CHAPTER XI

GEOGRAPHICAL DISTRIBUTION OF LAND MOLLUSCA (_continued_)--THE ETHIOPIAN, NEARCTIC, AND NEOTROPICAL REGIONS

=D. The Ethiopian Region=

The Ethiopian region includes the whole of Africa south of the Great Desert, and Southern Arabia, together with the outlying islands, excepting those of the Atlantidean province (p. 297).

Regarded as a whole, the Ethiopian is poorest in land Mollusca of all the tropical regions. And yet its characteristics are very remarkable. The entire _Achatina_ group is peculiar, and takes, especially in W. Africa, some curious forms (_Columna_, _Perideris_, _Pseudachatina_). Carnivorous Mollusca (_Ennea_, _Gibbus_, etc.) are highly developed, especially in the south and east, the largest known helicoid form (_Aerope_) being from Natal. In the possession of these types of the Agnatha, Africa is more closely related to the Australasian than to the Oriental region. The true _Cyclostoma_ are entirely peculiar to the region, but are absent from West Africa.

Fresh-water Mollusca are abundant and characteristic, especially in and near the Great Lakes. _Lanistes_, _Cleopatra_, and _Meladomus_, among the operculates, together with _Mutela_ and _Aetheria_ (Unionidae), _Galatea_ and _Fischeria_ (Cyrenidae), are peculiar.

In its negative, as well as its positive features, the Ethiopian region is markedly isolated. Helicidae and Naninidae are equally deficient, the former, indeed, attaining some numerical predominance in the extreme south, but the species are nearly all insignificant in size and colouring. It is only in Madagascar that _Helix_ asserts itself. _Arion_, _Limax_, _Hyalinia_, _Clausilia_, and a number of other genera abundant along the Mediterranean, are either altogether absent, or are very scantily represented. Land operculates, so characteristic of other tropical countries, are almost entirely wanting. If we disregard the Malagasy sub-region, there are scarcely forty species of land operculates on the whole African continent.

The Ethiopian region may be divided into three sub-regions: (1) the Central African; (2) the South African; (3) the Malagasy.

(1) =The Central African Sub-region= is bounded on the north by the Great Desert, on the east and west by the ocean, and on the south by a line roughly drawn between the mouth of the Orange River and Delagoa Bay; it also includes S. Arabia. No natural features exist which tend to break up this vast district into areas of independent zoological development. The absence of long and lofty mountain ranges, the enormous size of the great river basins, and the general uniformity of climate, equalise the conditions of life throughout. It will be convenient to break the sub-region up into provinces, but in most cases no precise line of demarcation can be laid down.

(_a_) The _Senegambian Province_ may be regarded as extending from the mouth of the Senegal River to Cape Palmas. Only 8 genera of land Mollusca are known, including 4 _Limicolaria_ and 3 _Thapsia_, with 1 small _Cyclophorus_. Fresh-water genera are abundant, and include most of the characteristic Ethiopian forms.

(_b_) The _West African Province_ extends from Cape Palmas to the mouth of the Congo, and is rich in Mollusca. The great _Achatina_, largest of land snails, whose shell sometimes attains a length of 6½ in., _Limicolaria_, _Perideris_, and _Pseudachatina_ are the characteristic forms. The Agnatha are represented by _Ennea_, _Streptaxis_, and _Streptostele_. _Rachis_ and _Pachnodus_, sub-genera of _Buliminus_, occur also on the east coast. A special feature is the development of several peculiar slug-like genera, _e.g._ _Oopelta_, perhaps a form of _Arion_; _Estria_, a slug with an external shell, akin to _Parmacella_; and _Aspidelus_, a form intermediate between _Helicarion_ and _Limax_. _Claviger_, a handsome group akin to _Cerithium_, is peculiar to the estuaries of West African rivers.

About sixteen species are known from the _Cameroons District_, but no peculiar genera occur. The _French Congo District_ has not yet been well explored. _Tomostele_, a genus allied to _Streptostele_, is peculiar, and _Pseudachatina_ attains its maximum.

[Illustration: FIG. 217.--_Columna flammea_ Müll., Princes I.]

_St. Thomas and Princes Is._, in the Gulf of Guinea, are well known. Princes I. has 22 species, 14 peculiar, and 2 common to St. Thomas only, one of the latter being the great sinistral _Achatina bicarinata_ Chem. The remarkable genus _Columna_ (Fig. 217) is peculiar, and _Streptostele_ (4 sp.) attains its maximum. Peculiar to St. Thomas are _Pyrgina_, a turreted form of _Stenogyra_; _Thyrophorella_, a sinistral form of _Zonites_; and _Atopocochlis_, a large bulimoid shell, whose true relationships are not yet known. _Homorus_, a group of _Achatina_ with an elongated spire, occurring also in the Angola District and on the east coast, has 4 species. No fresh-water species have as yet been discovered in either of the islands.

The _Angola and Benguela District_, extending from the Congo to the Cunene R., probably belongs to the West African Sub-region, but until its fauna is better known it is advisable to consider it apart. _Achatina_ continues abundant, but the other characteristic West African forms (_Pseudachatina_, _Streptostele_, _Perideris_) diminish or are absent altogether. No _Helix_ and only 1 _Cyclophorus_ occur.

_Ovampo_, _Damara_, and _Great Namaqualand_, lying between the Cumene and Orange rivers, seem to form a transition district between the West and South African faunas. _Helix_ reappears, while the characteristic West African genera are almost entirely wanting.

(_c_) The _East African Province_ extends from about Delagoa Bay to the Abyssinian shores of the Red Sea. In general out-line the province consists of a flat marshy district, extending inland for many miles from the sea; this is succeeded by rising ground, which eventually becomes a high table-land, often desolate and arid, whose line of slope lies parallel to the trend of the coast. The Mollusca are little known, and have only been studied in isolated districts, usually from the discoveries of exploring expeditions.

The _Mozambique District_, from Delagoa Bay to Cape Delgado, includes no genus which does not occur on the west coast, except _Cyclostoma_ (2 sp.). _Trochonanina_ (4 sp.), _Urocyclus_, a characteristic African slug (2 sp.), _Rachis_ (6 sp.), _Pachnodus_ (2 sp.), and _Achatina_ (5 sp.), are the principal groups.

[Illustration: FIG. 218.--_Urocyclus comorensis_ Fisch., Comoro Is.: =G=, Generative orifice; =M=, mucous gland; =O=, orifice leading to internal shell; =P=, pulmonary orifice; =T=, tentacles. (After Fischer.)]

The _Zanzibar District_, from Cape Delgado to the Somali country, has the same general features. _Meladomus_, a large sinistral _Ampullaria_, is characteristic, while _Cyclostoma_ (5 sp.) becomes more abundant. _Helix_ is still absent, but the carnivorous forms (_Streptaxis_ 2 sp., _Ennea_ 7 sp.) are rather numerous.

The _Somali District_ is characterised by operculate groups of the _Otopoma_ type (_Georgia_, _Rochebrunia_, _Revoilia_) whose generic value is rather doubtful. _Petraeus_, in an Arabian type, supplants _Rachis_ and _Pachnodus_. _Achatina_ is nearly wanting, but _Limicolaria_ has 9 species. A few _Helix_, said to be of the _Pisana_ group, occur.

The _District between the Great Lakes and the coast region_ is fairly well known through recent explorations, especially those associated with Emin Pasha. _Streptaxis_ (6 sp.) and _Ennea_ (24 sp.) are numerous, _Helix_ is wanting, and the Naninidae are represented by _Trochonanina_ (7 sp.), and other forms at present grouped under _Nanina_ or _Hyalinia_. On the high ground _Buliminus_, _Cerastus_, and _Hapalus_ replace, to some extent, the _Achatina_ and _Limicolaria_ of the marshy plains. Land operculates (_Cyclophorus_ 1, _Cyclostoma_ 8) are more numerous; among fresh-water genera we have _Lanistes_ (5 sp.), _Cleopatra_ (3 sp.), _Meladomus_ (1 sp.), and _Leroya_, a sinistral form with the facies of a _Littorina_. The characteristic African bivalves (_Mutela_, _Spatha_, etc.) are few in number.

(_d_) _Province of the Great Lakes._--The Mollusca of the four great lakes of Eastern Central Africa--Lakes Albert Nyanza (Luta Nzige, 2720 ft.), Victoria Nyanza (Oukéréwé, 3700 ft.), Nyassa (1520 ft.), and Tanganyika (2800 ft.)--are well known, and supply an interesting problem in distribution. Those of the three first mentioned lakes differ in no way from the rest of tropical Africa, but the Mollusca of Tanganyika include, in addition to the ordinary African element, a number of peculiar operculate genera, belonging principally to the Melaniidae and Hydrobiidae. Several of these possess a solidity of form and compactness of structure which is unusual in fresh-water genera, and has led to the belief, among some authorities, that they are the direct descendants of marine species, and that Tanganyika represents an ancient marine area. This view appears untenable. The Victoria Nyanza and Nyassa are part of the same system as Tanganyika, and it is not easy to see how, if Tanganyika were once an arm of the sea, they were not equally so, especially as they are several hundred miles nearer the Indian Ocean as at present defined. Nor, as will be seen from the figures given above, is there anything in the altitudes which would make us expect anything exceptional in Tanganyika. The similar case of L. Baikal must be compared (p. 290), where again a number of specialised forms of _Hydrobia_ occur.

Of the genera concerned, _Paramelania_ and _Nassopsis_ are forms of Melaniidae; _Tiphobia_ (Fig. 219), which is allied to _Paludomus_, is a compact shell with angulated spinose whorls; _Lacunopsis_, _Ponsonbya_, _Limnotrochus_, and _Tanganyicia_ are probably forms of _Lithoglyphus_, some, as their names denote, being of decidedly marine facies; _Syrnolopsis_ and _Turbonilla_ (?) look like Pyramidellidae, _Horea_ and _Reymondia_ like _Rissoina_; _Bourguignatia_ appears to belong to _Vivipara_, with which has now been merged the genus _Neothauma_. Recently discovered forms from the adjacent L. Mweru are evidently of kindred origin.

(_e_) The _Afro-Arabian Province_ includes Abyssinia, with S. Arabia, the African shores of the Gulf of Aden, and Socotra. The province contains a singular mixture of types. The high ground of Abyssinia stands like a lofty European island in the midst of a tropical plain, with Palaearctic genera flourishing like hardy northern plants on a mountain in low latitudes. _Helix_, _Vitrina_, and _Pupa_ abound, with a few _Clausilia_ and even a _Limax_. On the lower levels occur _Limicolaria_ (3 sp.), _Subulina_ (7 sp.), _Helicarion_, and _Homorus_, but land operculates are entirely wanting. Characteristic of the province as a whole are various forms of _Buliminus_, which in Socotra are represented by two peculiar sub-genera, _Achatinelloides_ and _Passamaiella_. In S. Arabia the mixture of types produces curious results: the _Helix_, _Clausilia_, and _Vitrina_ being Palaearctic, the _Limicolaria_ and all the operculates Ethiopian, while the single _Trochomorpha_ is Indian. Indian influence, indeed, comes out unmistakably throughout the province. Thus in Socotra there are two _Cyclotopsis_, in Abyssinia two _Africarion_ (closely related to the Indian _Girasia_), two _Microcystis_, and a _Glessula_, and in the Scioa district there is a _Sitala_. The fresh-water Mollusca of Socotra are Indian forms.

[Illustration: FIG. 219.--_Tiphobia Horei_ E. A. Smith, L. Tanganyika.]

[Illustration: FIG. 220.--Mollusca characteristic of L. Tanganyika: =A=, _Nassopsis nassa_ Woodw.; =B=, _Spekia zonata_ Woodw.; =C=, _Syrnolopsis lacustris_ E. A. Smith.]

[Illustration: FIG. 221.--_Achatina zebra_ Lam., S. Africa. × ½.]

(2) =The South African Sub-region.=--The principal characteristic of the Mollusca of S. Africa is the occurrence of numerous small species of Helicidae, belonging chiefly to the groups _Pella_, _Phasis_, _Dorcasia_, and _Sculptaria_, all of which are practically peculiar. Carnivorous genera are also prominent, _Ennea_ here attaining its maximum. _Rhytida_ (to which several species still regarded as _Pella_ belong) is common only to the S. Pacific and Australasia, and forms, with _Isidora_ among the fresh-water pulmonates, a remarkable link of connexion. _Aerope_, the largest of all helicoid carnivorous genera, and _Chlamydephorus_, a carnivorous slug with an internal shell, are peculiar. _Achatina_ is still abundant, but _Limicolaria_ is wanting. _Livinhacea_, a form with a continuous peristome, perhaps akin to _Bulimus_; _Apera_, a form of slug; and _Coeliaxis_, a genus perhaps akin to the Papuan and Queensland _Perrieria_, are all peculiar. The land operculates, which are not numerous, are of the East African type.

_Land Mollusca of the S. African Sub-region_

Chlamydephorus 1 Ennea 31 Aerope 5 Rhytida 3 Helicarion 3 Trochonanina 1 Trochozonites 1 Limax 1 Apera 1 Vitrina 7 Nanina 6 Conulus 2 Patula 2 Pella 44 Dorcasia 8 Phasis 1 Sculptaria 2 Helix (inc. sed.) 4 Rachis 1 Pachnodus 3 Buliminus (?) 4 Pupa 20 Vertigo 2 Achatina 18 Livinhacea 1 Stenogyra 4 Coeliaxis 1 Succinea 3 Vaginula 2 Cyclophorus 1 Cyclostoma 7 Cyclotus (?) 1 Blanfordia 1

_St. Helena._--The Molluscan fauna of St. Helena is perhaps the most puzzling, as regards its geographical affinities, of any in the world. It consists of 29 peculiar species of land Mollusca (fresh-water species being unknown), 19 of which are recently extinct, partly owing to the destruction of the forest, but are found in considerable abundance in a state of good preservation.[375] The genera are--

Hyalinia 1 Patula 4 (3 extinct) Endodonta 10 (7 extinct) Bulimulus 7 (5 extinct) Pachyotus 1 (extinct) Tomigerus (?) 1 (extinct) Pupa 2 (extinct) Succinea 3

The 5 genera which concentrate our attention are _Patula_, _Endodonta_, _Pachyotus_ (Fig. 222), _Tomigerus_, and _Bulimulus_, all of which appear utterly strange to an oceanic island in the middle of the S. Atlantic. _Patula_ and _Endodonta_ are essentially Polynesian forms, occurring abundantly on all the island groups in the Central Pacific. _Pachyotus_, _Tomigerus_ (assuming its correct identification), and _Bulimulus_ are all S. American forms, the two former being especially characteristic of Brazil. How this mixture of genera now confined to regions so widely distant, not only from St. Helena itself, but from one another, became associated here, is a problem obviously not easy of solution. The fauna is probably a remnant of a very ancient type, possibly at one time much more widely distributed. _Endodonta_ (an essentially insular form, like _Omphalotropis_) actually occurs on Fernando Noronha, off the Brazil coast, and we shall see how an Indian and even a Polynesian element is present off the eastern coasts of Africa.

_Ascension I._--One indigenous species, a so-called _Limax_, is all that has ever been discovered.

(3) =The Malagasy Sub-region= includes Madagascar with its attendant satellites Bourbon, Mauritius, and Rodriguez, and the Seychelles and Comoro groups. No land Mollusca are known from the Amirantes, the Chagos, or from Aldabra. The special characteristics of the sub-region are the great development of the carnivorous land Mollusca (_Ennea_, _Gibbus_), the occurrence of a considerable number of true Helicidae of great size and beauty, and the prominence of the genus _Cyclostoma_.

(_a_) _The Madagascan Province._--The land Mollusca of Madagascar, although as yet imperfectly known, possess a striking individuality. Two of the chief characteristics of the Ethiopian region are the paucity of its land operculate and of its _Helix_ fauna; Madagascar is especially distinguished by the rich development of both these groups. For size, colouring, and beauty of shape, the Helicidae of the two sub-genera _Ampelita_ and _Helicophanta_ rival, if they do not surpass, any in the world. They are quite peculiar to this sub-region, not a trace of them occurring on the Mascarenes, Seychelles, or even on the Comoros. _Helicophanta_ is distinguished by the enormous size of its embryonic shell, which persists in the adult (Fig. 223), and in this respect the group appears to be related to _Acavus_ (Ceylon, Fig. 204) and _Panda_ (N.E. Australia). As is usual when _Helix_ is well developed, _Nanina_ (about 12 sp.) is proportionately scanty.

The African _Bulimini_ (_Pachnodus_ and _Rachis_) are represented by two species, but _Achatina_, so abundant on the mainland, is scarce. Two other groups of _Buliminus_, _Leucotaenia_ and _Clavator_, are peculiar. The presence of a single _Kaliella_, specifically identical with a common Indian form, is very remarkable.

_Cyclostoma_ proper, of which Madagascar is the metropolis, is richly developed (54 sp.). Many of the species are of great size and of striking beauty of ornamentation. Unlike its Helicidae, this genus is not restricted to Madagascar; several species occur on the mainland, 6 on the Comoros, one on the Seychelles, and 16 in Mauritius. The sub-genera _Acroptychia_ and _Hainesia_ are peculiar.

[Illustration: FIG. 222.--_Pachyotus auris vulpina_ Desh., St. Helena (sub-fossil).]

[Illustration: FIG. 223.--_Helix_ (_Helicophanta_) _Souverbiana_ Fisch., Madagascar, showing embryonic shell. × ⅔.]

[Illustration: FIG. 224.--_Cyclostoma campanulatum_ Pfr., Madagascar.]

The fresh-water Mollusca of Madagascar contain further traces of Indian relationship. Thus we find two species of _Paludomus_, a genus whose metropolis is Ceylon, India, and Further India, and which is barely represented on the Seychelles and in the Somali district. _Melanatria_, which is peculiar to Madagascar, has its nearest affinities in the Cingalese and East Indian faunas. Several of the _Melania_ and the two _Bithynia_ are of a type entirely wanting in Africa, but common in the Indo-Malay sub-region. Not a single one of the characteristic African fresh-water bivalves (_Mutela_, _Spatha_, _Aetheria_, _Galatea_, etc.) has been found in Madagascar. On the other hand, certain African Gasteropoda, such as _Cleopatra_ and _Isidora_, occur, indicating, in common with the land Mollusca, that an ultimate land connexion with Africa must have taken place, but at an immeasurably remote period.

_Land and Fresh-water Mollusca of Madagascar_

Ennea 9 Urocyclus 2 Helicarion (?) 1 Macrocyclis (?) 1 Kaliella 1 Nanina (inc. sed.) 9 Ampelita 35 Helicophanta 17 Pachnodus 2 Rachis 2 Leucotaenia 2 Clavator 2 Achatina 3 Opeas 2 Subulina 3 Vaginula 4 Limnea 2 Planorbis 3 Isidora 3 Melania 7 Melanatria 4 Paludomus 2 Vivipara 1 Bithynia 2 Cleopatra 2 Ampullaria 6 Cyclophorus 2 Cyclotus (?) 1 Cyclostoma 54 Otopoma 5 Lithidion 1 Acroptychia 3 Hainesia 3 Unio 1 Corbicula 2 Sphaerium 1 Pisidium 1

_The Comoro Islands._--This isolated group possesses about 100 species, almost all of which are peculiar. The principal feature is the rich development of _Ennea_ (30 sp.). On the whole the group shows more relationship to Madagascar than to the mainland. Thus we have six species of true _Cyclostoma_, and only one _Achatina_, while among the fresh-water genera is _Septaria_, which is characteristic of the whole Malagasy Sub-region, but is absent from the mainland. The Helicidae are all of insignificant size. Peculiar to the group is the remarkable genus _Cyclosurus_ (Fig. 152, p. 247).

(_b_) _The Mascarene Province_ (Mauritius, Bourbon, Rodriguez, and the Seychelles).--The percentage of peculiar species, which is very high, can only be paralleled in the case of some of the West Indian islands, and sufficiently attests the extreme isolation of the group from Madagascar. We have--

Fresh-water Peculiar to Total sp. Land sp. sp. Peculiar group

Mauritius 113 104 9 78 102 (90 p.c.) Bourbon 45 40 5 19 38 (84 p.c.) Rodriguez 23 19 4 15 21 (95 p.c.) Seychelles 34 27 7 24 30 (90 p.c.)

The Mollusca of the group exhibit three distinct elements, the Indigenous, the Madagascan, and the Indian and Australasian.

The genus _Pachystyla_ (Naninidae) is quite peculiar, forming the main portion of the land snails proper. It attains its maximum in Mauritius (17 sp.), with 5 sp. in Bourbon and one sub-fossil sp. in Rodriguez, while in the Seychelles it is absent. But the principal feature of the Mascarene group is the extraordinary development of the carnivorous genus _Gibbus_, which has 27 sp. in Mauritius, 8 in Bourbon, 4 in Rodriguez; in the Seychelles, it is replaced by _Edentulina_ and _Streptostele_. The principal link with Madagascar is found in a part of the operculate land fauna. _Cyclostoma_ is present (with _Otopoma_) in several fine living forms, and the number of sub-fossil species is a clear indication that this group was, not long ago, much more abundant, for of the 16 _Cyclostoma_ known from Mauritius 10 are sub-fossil. The operculates form a decided feature of the land fauna; thus in Mauritius there are 32 species, or more than 28 per cent of the whole.

[Illustration: FIG. 225.--Characteristic Mauritian land shells: =A=, _Gibbus palanga_ Fér.; =A´=, young of same; =B=, _Gibbus lyonetianus_ Pall.]

Indian and Australasian affinities are unmistakably present. Thus _Omphalotropis_, a genus characteristic of _small_ islands, is profusely represented, but it does not occur in Madagascar or Africa. Two _Helicina_ (Mauritius and Seychelles) and a single _Leptopoma_ (possibly a _Leptopomoides_) are also of eastern relationship. _Cyclotopsis_, _Cyathopoma_, and _Geostilbia_ are markedly Indian genera. _Microcystis_, _Patula_, and _Tornatellina_ are Polynesian. _Hyalimax_--and this is a very striking fact--occurs nowhere else but in the Andamans and Nicobars, and on the Aracan coast. The nearest relation to the Seychelles _Mariaella_ appears to be the Cingalese _Tennentia_. Not a single representative of these eleven genera has been found even in Madagascar.

The fresh-water Mollusca (omitting the Neritidae) are: Mauritius 9 species, Bourbon 5, Rodriguez 4, Seychelles 6, with only 15 species in all. The one _Planorbis_ and the _Vivipara_, the _Paludomus_ and two of the _Melania_ are of Indian types. The _Lantzia_ (peculiar to Bourbon) is probably allied to the Indian _Camptonyx_. Owing to the paucity of permanent streams, no fresh-water bivalves occur. Among the Neritidae is a single _Septaria_, a genus which, though occurring in Madagascar, is entirely strange to Africa, and is abundant in the Oriental and Australasian regions.

It would seem probable that when the closer connexion which at one time undoubtedly existed between India and Eastern Africa began to be less continuous,[376] the Mascarene group was first severed from what ultimately became Madagascar, while the Seychelles, and perhaps the Comoros, still continued united to it. The Comoros, which lack the great _Helices_, separated off from Madagascar first, while the Seychelles continued in more or less direct union with that island sufficiently long to receive the progenitors of _Stylodonta_ (a peculiar group of _Helix_), but became disunited at an exceedingly remote period.

=E. The Nearctic Region=

The southern boundary of this region may be regarded as roughly corresponding to that of the United States, _i.e._ Lower California and Mexico are excluded. The southern portion of Florida belongs to the Antillean sub-region.

The principal characteristic of the Nearctic Region is the remarkable poverty of its land Mollusca. No district in the world of equal extent is so poor in genera, while those which occur are generally of small size, with scarcely anything remarkable either in colouring or form. The elongated land shells (_Clausilia_, _Buliminus_), so characteristic of Europe, are entirely wanting, but a few _Bulimulus_, of Neotropical origin, penetrate Texas, and from the same sources come a few species of _Glandina_ (as far north as S. Carolina), _Holospira_ (Texas), and _Helicina_.

The region falls into two well-marked sub-regions, the N. American and the Californian, with the Rocky Mountain district as a sort of debatable ground between them. The Californian sub-region consists of the narrow strip of country between the Sierra Nevada, the Cascade Mountains and the coast-line, from San Diego to Alaska; the N. American sub-region consists of the remainder of the region.

(1) =The N. American Sub-region.=--The Carnivorous genera are represented solely by the few _Glandina_ mentioned above, and by the indigenous genus _Selenites_, a form midway between _Testacella_ and _Limax_, whose metropolis is on the Pacific slope, but which spreads eastward into the Antilles. Among the Limacidae, _Limax_ is common to both sub-regions, but _Tebennophorus_ (4 sp., 3 of which belong to the genus _Pallifera_), a genus found also in China and Siam, and _Vitrinozonites_ do not occur in the Californian. _Hyalinia_ (_Zonites_) is fairly abundant, especially in the groups _Mesomphix_ and _Gastrodonta_ (peculiar to this sub-region), and _Hyalinia_ proper. _Patula_ is well represented. The Helicidae belong principally to the groups _Mesodon_, _Stenotrema_, _Triodopsis_, _Polygyra_, and _Strobila_, only 6 of which, out of a total of 84, reach the Pacific slope. Land operculates are conspicuous for their almost complete absence (see map, _frontispiece_).

[Illustration: FIG. 226.--Characteristic North American Mollusca. =A=, _Helix_ (_Mesodon_) _palliata_ Say, Ohio. =B=, _Helix_ (_Polygyra_) _cereolus_ Mühlf., Texas. =C=, _Patula alternata_ Say, Tennessee.]

The poverty of the land fauna is atoned for by the extraordinary abundance and variety of the fresh-water genera. A family of operculates, the Pleuroceridae, with 10 genera and about 450 species, is quite peculiar, a few stragglers only reaching Central America and the Antilles. The nucleus of their distribution is the Upper Tennessee River with its branches, and the Coosa River. They appear to dislike the neighbourhood of the sea, and are never found numerously within 100 miles of it. They adhere to stones in rapid water, and differ from the Melaniidae of the Old World and of S. America in the absence of a fringe to the mantle and in being oviparous. They do not occur north of the St. Lawrence River, or north of U.S. territory in the west, or in New England. Three-quarters of all the known species inhabit the rough square formed by the Tennessee River, the Mississippi, the Chattahoochee River, and the Gulf of Mexico. The Mississippi is a formidable barrier to their extension, and a whole section (_Trypanostoma_, with the four genera _Io_, _Pleurocera_, _Angitrema_, and _Lithasia_) does not occur west of that river. The Viviparidae are also very largely developed, the genera _Melantho_, _Lioplax_, and _Tulotoma_ being peculiar. The Pulmonata are also abundant, while the richness of the Unionidae may be gathered from the fact that Wetherby states[377] that in 1874 no less than 832 species in all had been described.

The entire Mississippi basin is inhabited by a common assemblage of Unionidae, and a considerable number of the species are distributed over the whole of this area, Texas, and parts of E. Mexico. Some species have spread out of this area into Michigan, Canada, the Red River, and Hudson’s Bay district, and even into streams in New York which drain into the Atlantic. An entirely different set of forms occupy the great majority of the rivers falling into the Atlantic, the Appalachian Mountains acting as an effective barrier between the two groups of species, which appear to mingle below the southern end of the range. In many cases Unionidae seem to have no difficulty in migrating from river to river, if the distance is not extreme; they probably are carried across overflowed districts in time of flood.[378]

[Illustration: FIG. 227.--_Helix_ (_Arionta_) _fidelis_ Gray, Oregon.]

(2) =The Californian Sub-region= is markedly distinct from the rest of N. America. The characteristic sombre Helices of the Eastern States are almost entirely wanting, and are replaced by _Arionta_ (20 sp.), a larger and more varied group, which may have some affinity to Chinese forms. _Glyptostoma_ (1 sp.) is also peculiar. _Selenites_ here has its metropolis, and _Pristiolma_ is a remarkable group of small _Hyalinia_ (_Zonites_), but the larger forms of the Eastern States are wanting. Several remarkable and quite peculiar forms of slug occur, namely, _Ariolimax_ (whose nearest relation is _Arion_), _Prophysaon_, _Hemphillia_, and _Binneya_. There are no land operculates.

Not more than 15 to 20 species of the Pleuroceridae (sect. _Goniobasis_) occur west of the Rocky Mountains, and only a single _Unio_, 5 _Anodonta_, and 1 _Margaritana_, which is common to New England. _Pompholyx_ is a very remarkable ultra-dextral form of _Limnaea_, apparently akin to the _Choanomphalus_ of L. Baikal. _Bithynia_, absent from the Eastern States, is represented by two species. The general indications are in favour of the Californian fauna having migrated from an Old World source after the upheaval of the Sierras; the American fauna, on the other hand, is purely indigenous, with no recent Old World influence at all.

_Land Mollusca of the Nearctic Region_

Glandina 4 Selenites 6 Limax 4 Vitrina 4 Vitrinozonites 1 Mesomphix 15 Hyalinia 22 Conulus 1 Gastrodonta 9 Pristiloma 2 Tebennophorus 4 Ariolimax 6 Prophysaon 2 Hemphillia 1 Binneya 1 Patula 18 Punctum 2 Arionta 20 Praticola 2 Glyptostoma 1 Mesodon 27 Stenotrema 11 Triodopsis 21 Polygyra 23 Polygyrella 2 Gonostoma 1 Vallonia 1 Strobila 2 Pupa 18 Vertigo 8 Holospira 2 Cionella 1 Bulimulus 6 Macroceramus 1 Succinea 21 Vaginulus 1 Helicina 2

=F. The Neotropical Region=

The land Mollusca of the Neotropical Region stand in complete contrast to those of the Nearctic. Instead of being scanty, they are exceedingly abundant; instead of being small and obscure, they are among the largest in size, most brilliant in colour, and most singular in shape that are known to exist. At the same time they are, as a whole, isolated in type, and exhibit but little relation with the Mollusca of any other region.

The most marked feature is the predominance of the peculiar genera _Bulimus_ and _Bulimulus_, the centre of whose development appears to lie in Peru, Ecuador, and Bolivia, but which diminish, both in numbers and variety of form, in the eastern portion of the region. In the forests of Central America, Venezuela, and Ecuador, and, to a lesser degree, in those of Peru and Brazil, occurs the genus _Orthalicus_, whose tree-climbing habits recall the _Cochlostyla_ of the Philippines. These three groups of bulimoid forms constitute, as far as the mainland is concerned, the preponderating mass of the land Mollusca. _Helix_ proper is most strongly developed in the Greater Antilles, which possess several peculiar groups of great beauty. In Central America _Helix_ is comparatively scarce, but in the northern portions of the continent several fine genera (_Labyrinthus_, _Isomeria_, _Solaropsis_) occur, which disappear altogether towards the south.

Carnivorous land Mollusca are, so far as Central America is concerned, more highly developed than in any other quarter of the world,

## particularly in the genera _Glandina_ and _Streptostyla_. These genera

also penetrate the northern portions of the continent, _Glandina_ reaching as far as Ecuador, and _Streptostyla_ as far as Peru. The Greater Antilles have also characteristic forms of these genera. _Streptaxis_ is tolerably abundant all over tropical South America, and is the one pulmonate genus which shows any affinity with the African fauna.

The slugs are exceedingly scarce. _Vaginula_ occurs throughout, and is the only genus in any sense characteristic.

_Clausilia_, in the sub-genus _Nenia_, occurs along the Andean chain from the extreme north (but not in Central America) as far south as Bolivia. It has in all probability made its way into S. America in exceedingly remote ages from its headquarters in Eastern Asia. No species survives in N. America, and a single straggler is found in Porto Rico. The genera _Macroceramus_, _Cylindrella_, and _Strophia_, are characteristic West Indian forms, which are only slightly represented on the mainland. _Homalonyx_, a curious form akin to _Succinea_, is peculiar to the region.

[Illustration: FIG. 228.--_Homalonyx unguis_ Fér., Demerara. _sh_, Shell (shown also separate); _p.o_, pulmonary orifice.]

Land operculates attain a most extraordinary development in the Greater Antilles, and constitute, in some cases, nearly one-half of the whole Molluscan fauna. Several groups of the Cyclostomatidae find their headquarters here, and some spread no farther. On the mainland this prominence does not continue. West Indian influence is felt in Central America and on the northern coast district, and some Antillean genera make their way as far as Ecuador. The whole group entirely disappears in Chili and Argentina, becoming scarce even in Brazil.

Among the fresh-water operculates, _Ampullaria_ is abundant, and widely distributed. _Vivipara_, so characteristic of N. America, is entirely absent. _Chilina_, a remarkable fresh-water pulmonate, akin to _Limnaea_, is peculiar to Chili, Patagonia, and Southern Brazil, but is not found in the tropical portion of the continent. Of the fresh-water Pelecypoda _Mycetopus_, _Hyria_, _Castalia_, _Leila_, and _Mülleria_ are peculiar forms, akin to the Unionidae.

(1) =The Antillean Sub-region= surpasses all other districts in the world in respect of (1) extraordinary abundance of species, (2) sharp definition of limits as a whole, (3) extreme localisation of the fauna of the separate islands. The sub-region includes the whole of the half-circle of islands from the Bahamas to Grenada, together with the extreme southern end of the peninsula of Florida, which was once, no doubt, a number of small islands like the Bahamas. Trinidad, and probably Tobago, although containing an Antillean element, belong to the mainland of S. America, from which they are only separated by very shallow water.

The sub-region appears to fall into four provinces:--

(_a_) Cuba, the Bahamas, and S. Florida; (_b_) Jamaica; (_c_) San Domingo (Haiti), Porto Rico, and the Virgin Is., with the Anguilla and St. Bartholomew group; (_d_) the islands from Guadeloupe to Grenada. The first three provinces contain the mass of the characteristic Antillean fauna, the primary feature being the extraordinary development of the land operculates, which here reaches a point unsurpassed in any other quarter of the globe. The relative numbers are as follows:--

Cuba Jamaica San Domingo Porto Rico

Inoperculate 362 221 152 75 Operculate 252 242 100 23

It appears, then, that the proportion of operculate to inoperculate species, while very high in Cuba (about 41 per cent of the whole), reaches its maximum in Jamaica (where the operculates are actually in a majority), begins to decline in San Domingo (about 40 per cent), and continues to do so in Porto Rico, where they are not more than 24 per cent of the whole. These operculates almost all belong to the families Cyclostomatidae and Helicinidae, only two genera (_Aperostoma_ and _Megalomastoma_) belonging to the _Cyclophorus_ group. Comparatively few genera are absolutely peculiar to the islands, one or two species of most of them occurring in Central or S. America, but of the several hundreds of operculate species which occur on the islands, not two score are common to the mainland.

[Illustration:

Map to illustrate the GEOGRAPHICAL DISTRIBUTION of the Land Mollusca of the WEST INDIES.

_The red line marks the 100 fathom line._

London: Macmillan and C^ọ. _London: Stanford’s Geog^ḷ Estab^ṭ._]

The next special feature of the sub-region is a remarkable development of peculiar sub-genera of _Helix_. In this respect the Antilles present a striking contrast to both Central and S. America, where the prime feature of the land Pulmonata is the profusion of _Bulimus_ and _Bulimulus_, and _Helix_ is relatively obscured. No less than 14 sub-genera of _Helix_, some of which contain species of almost unique beauty and size, are quite peculiar to the Greater Antilles, and some are peculiar to individual islands.

Here, too, is the metropolis of _Cylindrella_ (of which there are 130 species in Cuba alone), a genus which just reaches S. America, and has a few species along the eastern sea-board of the Gulf of Mexico. _Macroceramus_ and _Strophia_ are quite peculiar; the former, a genus allied to _Cylindrella_, which attains its maximum in Cuba and San Domingo, is scarcely represented in Jamaica, and disappears south of Anguilla; the latter, a singular form, resembling a large _Pupa_ in shape, which also attains its maximum in Cuba, is entirely wanting in Jamaica, and has its last representative in S. Croix. One species irregularly occurs at Curaçao.

The carnivorous group of land Mollusca are represented by several peculiar forms of _Glandina_, which attain their maximum in Jamaica and Cuba, but entirely disappear in the Lesser Antilles.

A certain number of the characteristic N. American genera are found in the Antillean Sub-region, indicating a former connexion, more or less intimate, between the W. Indies and the mainland. The genera are all of small size. The characteristic N. American _Hyalinia_ are represented in Cuba, San Domingo, and Porto Rico; among the _Helicidae_, _Polygyra_ reaches Cuba, but no farther, and _Strobila_ Jamaica. The fresh-water Pulmonata are of a N. American type, as far as the Greater Antilles are concerned, but the occurrence of _Gundlachia_ (Tasmania and Trinidad only) in Cuba is an unexplained problem at present. Unionidae significantly occur only at the two ends of the chain of islands, not reaching farther than Cuba (_Unio_ 3 sp.) at one end, and Trinidad (which is S. American) at the other.

A small amount of S. American influence is perceptible throughout the Antilles, chiefly in the occurrence of a few species of _Bulimulus_ and _Simpulopsis_. The S. American element may have strayed into the sub-region by three distinct routes: (1) by way of Trinidad, Tobago, and the islands northward; (2) by a north-easterly extension of Honduras towards Jamaica, forming a series of islands of which the Rosalind and Pedro banks are perhaps the remains; (3) by a similar approximation of the peninsula of Yucatan and the western extremity of Cuba. Central America is essentially S. American in its fauna, and the characteristic genera of Antillean operculates which occur on its eastern coasts are sufficient evidence of the previous existence of a land connexion more or less intimate (see map).

(_a_) _Cuba_ is by far the richest of the Antilles in land Mollusca, but it must be remembered that it is also much better explored than San Domingo, the only island likely to rival it in point of numbers. It contains in all 658 species, of which 620 are land and 38 fresh-water, the land operculates alone amounting to 252.

Carnivorous genera form but a small proportion of the whole. There are 18 _Glandina_ (which belong to the sections _Varicella_ and _Boltenia_) and 4 _Streptostyla_, the occurrence of this latter genus being peculiar to Cuba and Haiti (1 sp.) among the Antilles, and associating them closely with the mainland of Central America, where _Streptostyla_ is abundant. These two genera alone represent the Agnatha throughout the sub-region.

There are no less than 84 species of _Helix_, belonging to 12 sub-genera. Only one of these (_Polymita_) is quite peculiar to Cuba, but of 7 known species of _Jeanerettia_ and 8 of _Coryda_, 6 and 7 respectively are Cuban. _Thelidomus_ has 15 species (Jamaica 3, Porto Rico 3); _Polydontes_ has 3, the only other being from Porto Rico; _Hemitrochus_ has 12 (Jamaica 1, Bahamas 6); _Cysticopsis_ 9 (Jamaica 6); _Eurycampta_ 4 (Bahamas 1).

The Cylindrellidae find their maximum development in Cuba. As many as 34 _Macroceramus_ occur (two-thirds of the known species), and 130 _Cylindrella_, some of the latter being most remarkable in form (see Fig. 151, B, p. 247).

The land operculates belong principally to the families Cyclostomatidae and Helicinidae. Of the former, Cuba is the metropolis of _Ctenopoma_ and _Chondropoma_, the former of which includes 30 Cuban species, as compared with 1 from San Domingo and 2 from Jamaica. _Megalomastoma_ (Cyclophoridae) is also Haitian and Porto Rican, but not Jamaican. _Blaesospira_, _Xenopoma_, and _Diplopoma_ are peculiar. The Helicinidae consist mainly of _Helicina_ proper (58 sp.), which here attains by far its finest development in point of size and beauty, and of _Eutrochatella_ (21 sp.), which is peculiar to the three great islands (Jamaica 6 sp., San Domingo 6 sp.).

The _Bahamas_, consisting in all of more than 700 islands, are very imperfectly known, but appear to be related partly to Cuba, partly to San Domingo, from each of which they are separated by a narrow channel of very deep water. They are certainly not rich in the characteristic groups of the Greater Antilles. The principal forms of _Helix_ are _Plagioptycha_ (6 sp.), common with San Domingo, and _Hemitrochus_ (6 sp.), common with Cuba. _Strophia_ is exceedingly abundant, but _Cylindrella_, _Macroceramus_, and _Glandina_ have but few species. There are a few species of _Ctenopoma_, _Chondropoma_, and _Cistula_, while a single _Schasicheila_ (absent from the rest of the sub-region) forms a link with Mexico.

[Illustration: FIG. 229.--Characteristic Cuban Helices. =A=, _Polydontes imperator_ Montf. =B=, _Caracolus rostrata_ Pfr. =C=, _Polymita muscarum_ Lea.]

_Southern Florida_, with one or two species each of _Hemitrochus_, _Cylindrella_, _Macroceramus_, _Strophia_, _Ctenopoma_, and _Chondropoma_, belongs to this province.

(_b_) _Jamaica._--The land Mollusca of Jamaica are, in point of numbers and variety, quite unequalled in the world. There are in all as many as 56 genera and more than 440 species, the latter being nearly all peculiar. The principal features are the Glandinae, the Helicidae, and the land operculates. The Glandinae belong principally to the sub-genera _Varicella_, _Melia_, and _Volutaxis_, _Streptostyla_ being absent, although occurring in Cuba and San Domingo. There are 10 genera of _Helix_, of which _Pleurodonta_ is quite peculiar, while _Sagda_ (13 sp.) is common only with S.W. San Domingo (2 sp.), and _Leptoloma_ (8 sp.) only with Cuba (1 sp.). The single _Strobila_ seems to be a straggler from a N. American source. _Macroceramus_ has only 2 species as against 34 in Cuba, and of _Cylindrella_, in which Cuba (130 sp.) is so rich, only 36 species occur. The genus _Leia_, however (14 sp.), is all but peculiar, occurring elsewhere only in the neighbouring angle of San Domingo, which is so closely allied with Jamaica. The complete absence of _Strophia_ is remarkable.

[Illustration: FIG. 230.--Characteristic Jamaican and Haitian Mollusca: =A=, _Sagdae pistylium_ Müll., Jamaica; =B=, _Chondropoma salleanum_ Pfr., San Domingo; =C=, _Eutrochatella Tankervillei_ Gray, Jamaica; =D=, _Cylindrella agnesiana_ C. B. Ad., Jamaica.]

The land operculates form the bulk of the land fauna, there being actually 242 species, as against 221 of land Pulmonata, a proportion never again approached in any part of the world. As many as 80 of these belong to the curious little genus _Stoastoma_, which is all but peculiar to the island, one species having been found in San Domingo, and one in Porto Rico. _Geomelania_ and _Chittya_, two singular inland forms akin to _Truncatella_, are quite peculiar. _Alcadia_ reaches its maximum of 14 species, as against 4 species in San Domingo and 9 species in Cuba, and _Lucidella_ is common to San Domingo only; but, if _Stoastoma_ be omitted, the Helicinidae generally are not represented by so many or by so striking forms as in Cuba, which has 90 species, as against Jamaica 44, and San Domingo 35.

(_c_) _San Domingo_, although not characterised by the extraordinary richness of Cuba and Jamaica, possesses many specially remarkable forms of land Mollusca, to which a thorough exploration, when circumstances permit, will no doubt make important additions. From its geographical position, impinging as it does on all the islands of the Greater Antilles, it would be expected that the fauna of San Domingo would not exhibit equal signs of isolation, but would appear to be influenced by them severally. This is exactly what occurs, and San Domingo is consequently, although very rich in peculiar species, not equally so in peculiar genera. The south-west district shows distinct relations with Jamaica, the Jamaican genera _Leia_, _Stoastoma_, _Lucidella_, and the _Thaumasia_ section of _Cylindrella_ occurring here only. The north and north-west districts are related to Cuba, while the central district, consisting of the long band of mountainous country which traverses the island, contains the more characteristic Haitian forms.

The Helicidae are the most noteworthy of the San Domingo land Mollusca. The group _Eurycratera_, which contains some of the finest existing land snails, is quite peculiar, while _Parthena_, _Cepolis_, _Plagioptycha_, and _Caracolus_ here reach their maximum. The Cylindrellidae are very abundant, but no section is peculiar. Land operculates do not bear quite the same proportion to the Pulmonata as in Cuba and Jamaica, but they are well represented (100 to 152); _Rolleia_ is the only peculiar genus.

The relations of San Domingo to the neighbouring islands are considerably obscured by the fact that they are well known, while San Domingo is comparatively little explored. To this may perhaps be due the curious fact that there are actually more species common to Cuba and Porto Rico (26) than to Porto Rico and San Domingo. Cuba shares with San Domingo its small-sized _Caracolus_ and also _Liguus_, but the great _Eurycratera_, _Parthena_, and _Plagioptycha_ are wholly wanting in Cuba. The land operculates are partly related to Cuba,

## partly to Jamaica, thus _Choanopoma_, _Ctenopoma_, _Cistula_, _Tudora_,

and many others, are represented on all these islands, while the Jamaican _Stoastoma_ occurs on San Domingo and Porto Rico, but not on Cuba, and _Lucidella_ is common to San Domingo and Jamaica alone. An especial link between Jamaica and San Domingo is the occurrence in the south-west district of the latter island of _Sagda_ (2 sp.). The relative numbers of the genera _Strophia_, _Macroceramus_, and _Helicina_, as given below (p. 351), are of interest in this connexion.

_Porto Rico_, with Vièque, is practically a fragment of San Domingo. The points of close relationship are the occurrence of _Caracolus_, _Cepolis_, and _Parthena_ among the Helicidae, and of _Simpulopsis_, _Pseudobalea_, and _Stoastoma_. _Cylindrella_ and _Macroceramus_ are but poorly represented, but _Strophia_ still occurs. The land operculates (see the Table) show equal signs of removal from the headquarters of development. _Megalomastoma_, however, has some striking forms. The appearance of a single _Clausilia_, whose nearest relations are in the northern Andes, is very remarkable. _Gaeotis_, which is allied to _Peltella_ (Ecuador only), is peculiar.

[Illustration: FIG. 231.--Examples of West Indian Helices: =A=, _Helix_ (_Parthena_) _angulata_ Fér., Porto Rico; =B=, _Helix_ (_Thelidomus_) _lima_ Fér., Vièque; =C=, _Helix_ (_Dentellaria_) _nux denticulata_ Chem., Martinique.]

_Land Mollusca of the Greater Antilles_

Cuba. Jamaica. S. Domingo. Porto Rico.

Glandina 18 24 15 8 Streptostyla 4 ... 2 ... Volutaxis ... 11 (?) 1 ... Selenites 1 ... ... ... Hyalinia 4 11 5 6 Patula 5 1 ... ... Sagda ... 13 2 ... Microphysa 7 18 8 3 Cysticopsis 9 6 ... ... Hygromia (?) ... ... 3 ... Leptaxis (?) ... ... 1 ... Polygyra 2 ... ... ... Jeanerettia 6 ... ... 1 Euclasta ... ... ... 4 Plagioptycha ... ... 14 2 Strobila ... 1 ... ... Dialeuca ... 1 ... ... Leptoloma 1 8 ... ... Eurycampta 4 ... ... ... Coryda 7 ... ... ... Thelidomus 15 3 ... 3 Eurycratera ... ... 7 ... Parthena ... ... 2 2 Cepolis ... ... 3 1 Caracolus 8 ... 6 2 Polydontes 3 ... ... 1 Hemitrochus 12 1 ... ... Polymita 5 ... ... ... Pleurodonta ... 34 ... ... Inc. sed. 5 ... ... ... Simpulopsis ... ... 1 1 Bulimulus 3 3 6 7 Orthalicus 1 1 ... ... Liguus 3 ... 1 ... Gaeotis ... ... ... 3 Pineria 2 ... ... 1 Macroceramus 34 2 14 3 Leia ... 14 2 ... Cylindrella 130 36 35 3 Pseudobalea 2 ... 1 1 Stenogyra 6 7 (?) ... Opeas 8 (?) 4 6 Subulima 6 14 2 2 Glandinella 1 ... ... ... Spiraxis 2 (?) 2 1 Melaniella 7 ... ... ... Geostilbia 1 ... 1 ... Cionella 2 ... ... ... Leptinaria ... 1 ... 3 Obeliscus ... ... 1 2 Pupa 2 7 3 2 Vertigo 4 ... ... ... Strophia 19 ... 3 2 Clausilia ... ... ... 1 Succinea 11 2 5 3 Vaginula 2 2 2 1 Megalomastoma 13 ... 1 3 Neocyclotus 1 33(?) ... ... Licina 1 ... 3 ... Jamaicia ... 2 ... ... Crocidopoma ... 1 3 ... Rolleia ... ... 1 ... Choanopoma 25 12 19 3 Ctenopoma 30 2 1 ... Cistula 15 3 3 3 Chondropoma 57 (?) 19 4 Tudora 7 17 5 ... Adamsiella 1 12 ... ... Blaesospira 1 ... ... ... Xenopoma 1 ... ... ... Cistula 15 3 3 ... Colobostylus 4 13 5 ... Diplopoma 1 ... ... ... Geomelania ... 21 ... ... Chittya ... 1 ... ... Blandiella ... ... 1 ... Stoastoma ... 80 1 1 Eutrochatella 21 6 6 ... Lucidella ... 4 1 ... Alcadia 9 14 4 ... Helicina 58 16 24 9 Proserpina 2 4 ... ...

The Virgin Is., with St. Croix, Anguilla, and the St. Bartholomew group (all of which are non-volcanic islands), are related to Porto Rico, while Guadeloupe and all the islands to the south, up to Grenada (all of which are volcanic), show marked traces of S. American influence. St. Kitt’s, Antigua, and Montserrat may be regarded as intermediate between the two groups. St. Thomas, St. John, and Tortola have each one _Plagioptycha_ and one _Thelidomus_, while St. Croix has two sub-fossil _Caracolus_ which are now living in Porto Rico, together with one _Plagioptycha_ and one _Thelidomus_ (sub-fossil). The gradual disappearance of some of the characteristic greater Antillean forms, and the appearance of S. American forms in the Lesser Antilles, is shown by the following table:--

+-----------------------+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | | | | | | | | | | | | | | S | | | | | P | S | | | | | S | | G | | M | | | t | | | | | o | t | | S | | | t | | u | | a | S | | . | | | | | r | . | | t | | A | . | | a | D | r | t | B | | | T | | | t | | S | . | T | n | | A | d | o | t | . | a | V | G | r | | | o | T | t | | o | g | K | n | e | m | i | | r | i | r | i | | | | h | . | C | r | u | i | t | l | i | n | L | b | n | e | n | | | R | o | | r | t | i | t | i | o | n | i | u | a | c | n | i | | | i | m | J | o | o | l | t | g | u | i | q | c | d | e | a | d | | | c | a | a | i | l | l | ’ | u | p | c | u | i | o | n | d | a | | | o | s | n | x | a | a | s | a | e | a | e | a | s | t | a | d | | | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | +-----------------------+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | Bulimulus | 7 | 4 | 2 | 4 | 1 | 2 | 2 | 3 | 8 | 9 | 5 | 3 | 3 | 6 | 2 | 4 | | Cylindrella | 3 | 2 | 1 | 1 | 1 | . | . | . | 1 | . | 1 | 1 | 1 | . | . | 1 | | Macroceramus | 3 | 1 | 1 | . | 2 | 1 | . | . | . | . | . | . | . | . | . | | | Cyclostomatidae, etc. |23 | 4 | 1 | 5 | 1 | 1 | 1 | . | 4 | . | . | . | . | . | . | 1 | | Dentellaria | . | . | . | . | . | . | 1 | 1 | 8 | 5 |11 | 2 | 2 | . | 1 | 1 | | Cyclophorus | . | . | . | . | . | . | . | . | 1 | 2 | 2 | . | . | . | . | . | | Amphibulimus | . | . | . | . | . | . | . | . | 2 | 3 | 1 | . | . | . | . | . | | Homalonyx | . | . | . | . | . | . | . | . | 1 | 1 | . | . | . | . | . | . | +---------------------------------------------------------------------------------------+

(_d_) In Guadeloupe we find _Cyclophorus_, _Amphibulimus_, _Homalonyx_, and _Pellicula_, which are characteristic of S. America, and nearly all recur in Dominica and Martinique. These islands are the metropolis of _Dentellaria_, a group of _Helix_, evidently related to some of the forms developed in the Greater Antilles. Stragglers occur as far north as St. Kitt’s and Antigua, and there are several on the mainland as far south as Cayenne. Traces of the great _Bulimus_, so characteristic of South America, occur as far north as S. Lucia, where also is found a _Parthena_ (San Domingo and Porto Rico). Trinidad is markedly S. American; 55 species in all are known, of which 22 are peculiar, 28 are common to S. America (8 of these reach no farther north along the islands), and only 5 are common to the Antilles, but not to S. America. The occurrence of _Gundlachia_ in Trinidad has already been mentioned.

_The Bermudas_ show no very marked relationship either to the N. American or to the West Indian fauna. In common with the former they possess a _Polygyra_, with the latter (introduced species being excluded) one species each of _Hyalosagda_, _Subulina_, _Vaginula_, and _Helicina_, so that, on the whole, they may be called West Indian. The only peculiar group is _Poecilozonites_, a rather large and depressed shell of the _Hyalinia_ type.

(2) =The Central American Sub-region= may be regarded as extending from the political boundary of Mexico in the north to the isthmus of Panama in the south. It thus impinges on three important districts--the N. American, West Indian, and S. American; and it appears, as we should perhaps expect, that the two latter of these regions have considerably more influence upon its fauna than the former. Of the N. American Helicidae, _Polygyra_ is abundant in Mexico only, and two species of _Strobila_ reach N. Guatemala, while the Californian _Arionta_ occurs in Mexico. S. American Helicidae, in the sub-genera _Solaropsis_ and _Labyrinthus_, occur no farther north than Costa Rica. Not a single representative of any of the characteristic West Indian Helicidae occurs. _Bulimulus_ and _Otostomus_, which form so large a proportion of the Mollusca of Venezuela, Colombia, Ecuador, and Peru, together with _Orthalicus_, are abundant all over the region. Again, _Cylindrella_, _Macroceramus_, and some of the characteristic Antillean operculates, are represented, their occurrence being in most cases limited to the eastern coast-line and eastern slope of the central range.

Besides these external elements, the region is rich in indigenous genera. Central America is remarkable for an immense number of large carnivorous Mollusca possessing shells. There are 49 species of _Glandina_, the bulk of which occur in eastern and southern Mexico; 36 of _Streptostyla_ (S.E. Mexico and Guatemala, only 1 species reaching Venezuela and another Peru); 5 of _Salasiella_, 2 of _Petenia_, and 1 of _Strebelia_; the last three genera being peculiar. _Streptaxis_, fairly common in S. America, does not occur. _Velifera_ and _Cryptostracon_, two remarkable slug-like forms, each with a single species, are peculiar to Costa Rica. Among the especial peculiarities of the region are the giant forms belonging to the Cylindrellidae, which are known as _Holospira_, _Eucalodium_, and _Coelocentrum_ (Fig. 232). They are almost entirely peculiar to Mexico, only 7 out of a total of 33 reaching south of that district, and only 1 not occurring in it at all.

[Illustration: FIG. 232.--Examples of characteristic Mexican Mollusca: =A=, _Coelocentrum turris_ Pfr.; =B=, _Streptostyla Delattrei_ Pfr.]

The land operculates are but scanty. _Tomocyclus_ and _Amphicyclotus_ are peculiar, and _Schasicheila_, a form of _Helicina_, occurs elsewhere only in the Bahamas. _Ceres_ (see Fig. 18, C, p. 21) and _Proserpinella_, two remarkable forms of non-operculate Helicinidae (compare the Chinese _Heudeia_), are quite peculiar. _Pachychilus_, one of the characteristic fresh-water genera, belongs to the S. American (Melaniidae) type, not to the N. American (Pleuroceridae). Among the fresh-water Pulmonata, the _Aplecta_ are remarkable for their great size and beauty. In the accompanying table “Mexico” is to be taken as including the region from the United States border up to and including the isthmus of Tehuantepec, and “Central America” as the whole region south of that point.

_Land Mollusca of Central America_

Mexico only. Central Common to America both. only.

Strebelia 1 ... ... Glandina 33 13 3 Salasiella 4 ... 1 Streptostyla 18 12 6 Petenia ... 1 1 Limax ... 1 ... Velifera ... 1 ... Omphalina 10 1 1 Hyalinia 2 5 3 Guppya ... 8 3 Pseudohyalina 2 ... 2 Tebennophorus 1 ... ... Cryptostracon ... 1 ... Xanthonyx 4 ... ... Patula 3 ... 4 Acanthinula 1 2 2 Vallonia ... 1 ... Trichodiscus 2 2 3 Praticolella 1 ... 1 Arionta 3 ... ... Lysinoe 1 1 1 Oxychona 2 5 ... Solaropsis ... 2 ... Polygyra 14 1 2 Strobila 1 1 ... Labyrinthus ... 5 ... Otostomus 23 20 7 Bulimulus 6 5 2 Berendtia 1 ... ... Orthalicus 6 3 3 Pupa 1 1 1 Vertigo 1 ... ... Holospira 12 ... ... Coelocentrum 6 1 1 Eucalodium 15 ... 5 Cylindrella 6 4 ... Macroceramus 2 1 ... Simpulopsis 2 1 ... Caecilianella 1 ... ... Opeas 1 2 3 Spiraxis 8 2 1 Leptinaria ... 2 ... Subulina 2 3 4 Succinea 11 3 1 Vaginula 1 ... ... Aperostoma ... 4 1 Amphicyclotus 2 1 2 Cystopoma 2 ... ... Tomocyclus ... 1 2 Choanopoma 2 2 ... Chondropoma 2 11 ... Helicina 13 10 6 Schasicheila 2 ... 1 Ceres 2 ... ... Proserpinella 1 ... ...

(3) =The Colombian Sub-region= includes Colombia, New Grenada, Venezuela, Guiana, Ecuador, Peru, and Bolivia. It has been usual to separate off the two latter countries as forming a distinct “Peruvian” sub-region; but there is, as will be seen, absolutely no line to be drawn between the Mollusca of Peru and those of Ecuador; nor would one, on geographical considerations, expect to be able to draw such a line. A better method of subdivision, so far as the species of the whole eastern portion of the region are concerned, would be to group the Mollusca according to the altitude at which they occur, were it not that the evidence on this point is at present but fragmentary. We know, however, that all along the line of the Andes certain species, more

## particularly of _Bulimulus_, occupy their own zones of elevation, some

ascending as high as 10,000 feet above the sea, and never occurring on the plains.

In the northern portions of this sub-region, Central American and West Indian influence is felt to a certain extent. Thus there are eight _Glandina_ and one _Streptostyla_ in Venezuela and Colombia together with one or two species of _Cistula_, _Chondropoma_, _Proserpina_, and _Cylindrella_, while a single _Strophia_ (decidedly a straggler) occurs at Curaçao. In Demerara and Cayenne there are three or four species of _Dentellaria_. In Ecuador, however, _Glandina_ diminishes to three species, and in Peru disappears altogether, although one _Streptostyla_ occurs. Similarly the West Indian operculates are reduced to one _Chondropoma_ (Ecuador) and disappear entirely in Peru.

[Illustration: FIG. 233.--=A=, _Orthalicus Deburghiae_ Reeve, Ecuador; =B=, _Bulimus_ (_Pachyotus_) _egregius_ Jay, Brazil.]

The Helicidae are most abundant in the north and west, and are represented by several very striking sub-genera, some of which possess remarkably toothed apertures, and perhaps betray an ancestry common to some of the West Indian genera. Of these, _Labyrinthus_ has 12 species in Venezuela and Colombia, 5 in Ecuador, and 3 in Peru and Bolivia; _Isomeria_ 12 in Venezuela and Colombia, 20 in Ecuador, and 2 in Peru and Bolivia; _Salaropsis_ is represented in these countries by 6, 3, and 7 species, and _Systrophia_ by 4, 5, and 8 species respectively.

_Clausilia_--in the group _Nenia_--appears in some numbers along the Andes chain, the only other representative in the New World being the solitary species occurring at Porto Rico. There have been described, from Venezuela and Colombia 10 species, from Ecuador 5, and from Peru and Bolivia 12.

Another marked feature of the region is the occurrence of the Orthalicidae, in the two genera _Orthalicus_ and _Porphyrobaphe_. The latter of these magnificent forms is peculiar, while the former reaches Mexico, the West Indies, and Brazil. Ecuador, which contains 23 species, seems the metropolis of the group.

_Bulimus_ and _Bulimulus_, the former genus being peculiar to S. America and the adjacent islands, are largely represented, the former in the three groups _Borus_, _Dryptus_, and _Orphnus_. These attain their maximum in Peru, with 25 species, but Venezuela and Colombia have as many as 17. _Bulimulus_ has been subdivided into a number of groups, _e.g._ _Drymaeus_, _Mesembrinus_, _Thaumastus_, _Mormus_, _Scutalus_, with many others,--the exact scientific limits of which are not easily discernible. It must suffice here to state that Peru seems to be the headquarters of the group with about 190 species (which probably may well be reduced), Ecuador having about 70, and Venezuela and Colombia between 80 and 90.

[Illustration: FIG. 234.--_Rhodea gigantea_ Mouss., New Grenada.]

Two very remarkable forms belonging to the Pupidae, _Anostoma_ (Fig. 154, p. 248) and _Tomigerus_, occur in Venezuela, the metropolis. _Rhodea_, another very peculiar shell (Fig. 234), whose exact family position is uncertain, is peculiar to New Grenada. The land operculates are few in number, and in Bolivia almost disappear. They belong principally to _Neocyclotus_ (of which 11 species occur in Venezuela and Colombia) and _Helicina_ (10 species in the same district), besides the stragglers already mentioned from West Indian sources, and a few _Cyclophorus_. _Bourcieria_ is a form of _Helicina_ peculiar to Ecuador. _Ampullaria_, with _Ceratodes_, a peculiar planorbiform sub-genus, and _Hemisinus_, form the bulk of the fresh-water operculates.

_The Galapagos._--Thirty-four species of land Mollusca, all peculiar, are known from these islands; 25 of these are forms of _Bulimulus_. There are no Helicidae, one each of _Hyalinia_, _Leptinaria_, and _Helicina_, and two _Pupa_. The _Bulimulus_ are mostly of the group _Nesiotis_, and in their brown colour bear some outward resemblance to the dark _Achatinella_ of the Sandwich Is., living as they do mostly under scoriae on the ground, and not on trees. In type, however, they appear to be derived from Chili and Peru, rather than from the parts of S. America immediately contiguous. Another section (_Pleuropyrgus_ 2 sp.) closely resembles a marine _Chemnitzia_. The islands are all volcanic, and are probably not the result of subsidence; thus the existing species are not to be regarded as the relics of a more widespread fauna, but as a new set of inhabitants.

(4) =The Brazilian Sub-region.=--This immense district is very little known, except in the south, and it is consequently impossible to give any satisfactory account of its Mollusca. It is possible that eventually it will be found that it falls into provinces which correspond more or less to (_a_) the Amazon basin; (_b_) the mountainous district in the east, drained by the Tocantins and the San Francisco; (_c_) the Parana basin in the south central district; and (_d_) the Argentine or Pampas district in the extreme south. But at present the data are insufficient to establish any such subdivisions, whose existence, if proved, would have an important bearing on the problem of the coalescence of S. America into its present form.[379]

The Agnatha are represented by _Streptaxis_ alone (17 sp.). _Helix_ is rare, but includes the peculiar _Polygyratia_ (Fig. 150 A, p. 246), while _Labyrinthus_ (2 sp.), _Solaropsis_ (5 sp.), and _Systrophia_ are common with the Colombian Sub-region, and _Oxychona_ (4 sp.) with the Central American. _Bulimus_ has in all 36 species, the sub-genera _Pachyotus_ (Fig. 233) and _Strophochilus_ being peculiar. _Bulimulus_, though not so abundant as in Peru and Ecuador, has about 60 species, of which _Navicula_ (Fig. 235) is the most remarkable group. _Megaspira_ is peculiar. _Orthalicus_ has only 4 species, while _Tomigerus_ (4 sp.) and _Anostoma_ (3 sp.) are common with Venezuela. Land operculates are scarce, and appear to include only _Neocyclotus_, _Cyclophorus_, and _Helicina_.

In Argentina, which may probably rank as a separate province, the tropical forms greatly decrease, _Streptaxis_ being reduced to 2 species, and _Bulimus_ and _Bulimulus_ together to 40, while _Orthalicus_, the great Helices, and the land operculates disappear altogether. _Odontostomus_ (Fig. 236), a genus of the Pupidae, is abundant in the northern part of the province. Two or three species of _Chilina_ occur.

[Illustration: FIG. 235.--_Bulimulus_ (_Navicula_) _navicula_ Wagn., Brazil.]

[Illustration: FIG. 236.--_Odontostomus pantagruelinus_ Moric., S. Brazil. × ½.]

(5) =The Chilian Sub-region.=--The greater part of Chili, from its arid and rainless climate, is unfavourable to the existence of land Mollusca. _Bulimus_ (_Borus_) still has 3 or 4 species, and _Bulimulus_ (_Plectostylus_ 11, _Scutalus_ 9, _Peronaeus_ 7) is fairly abundant, but the profusion of the tropics is wanting. There are no carnivorous genera, and only two land operculates. A remarkable form of _Helix_ (_Macrocyclis_, Fig. 237) is quite peculiar, but the majority of the species belong to two rather obscure groups, _Stepsanoda_ and _Amphidoxa_. _Chilina_, a singularly solid form of _Limnaea_ (of which 8 sp., with a sub-genus _Pseudochilina_, occur in Chili), is peculiar to Chili, S. Brazil, and Patagonia. From the two islands of _Juan Fernandez_ and _Masafuera_, are known several _Helix_, of Chilian affinity, several curious _Succinea_, a _Homalonyx_, _Leptinaria_, and _Nothus_, and three species of _Tornatellina_, with the almost universal _Limax gagates_.

The question of the existence at some remote period of a Neantarctic continent, which formed a communication between the three great southern peninsulas of the world, is one on which the Mollusca may offer evidence. Von Ihering holds that an essential difference can be observed between certain of the Unionidae which inhabit S. America, Africa, and Australia with New Zealand, and those which inhabit Europe, Asia, and N. America, but the point can hardly be regarded as definitely established at present. Something perhaps may be made of the distribution of _Bulimus_ and _Bulimulus_. It seems difficult to explain the occurrence of sub-fossil _Bulimus_ on St. Helena except on some such lines as have been recently adduced to account for the presence of struthious birds in the Mascarenes, and possibly the form _Livinhacea_ may be a trace of the same element in S. Africa. Again, the _Liparus_ of S. and W. Australia, with the _Caryodes_ of Tasmania, and the _Leucotaenia_ and _Clavator_ of Madagascar (which all may be related to _Bulimus_), together with the _Placostylus_ of New Caledonia and the adjacent islands, reaching even to New Zealand, and perhaps even the _Amphidromus_ of Malaysia (which are more akin to _Bulimulus_), may be thought to exhibit, in some remote degree, traces of a common ancestry.

The land operculates give no help, and, of the carnivorous genera, _Rhytida_ is a marked link between Africa and Australia, while _Streptaxis_ is equally so between S. America and Africa. As regards fresh-water Gasteropoda, _Ampullaria_ is common to S. America and Africa, while _Isidora_ is common to Africa, Australia, and New Zealand, but is altogether absent from S. America. _Gundlachia_ occurs in Florida, Trinidad, and Tasmania, but has not been detected in Africa. It must be concluded, therefore, that the present state of the evidence which the Mollusca can afford, while exhibiting certain curious points of relationship between the three regions in question, is insufficient to warrant any decided conclusion.

[Illustration: FIG. 237.--_Macrocyclis laxata_ Fér., Chili.]

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