CHAPTER XII

DISTRIBUTION OF MARINE MOLLUSCA--DEEP-SEA MOLLUSCA AND THEIR CHARACTERISTICS

Marine Mollusca may be divided roughly into Pelagic and non-Pelagic genera. To the former division belong all Pteropoda and Heteropoda, and a large number of Cephalopoda, together with a very few specialised forms of Gasteropoda (_Ianthina_, _Litiopa_, _Phyllirrhoe_, etc.). Pelagic Mollusca appear, as a rule, to live at varying depths below the surface during the day, and to rise to the top only at night. The majority inhabit warm or tropical seas, though some are exceedingly abundant in the Arctic regions; _Clione_ and _Limacina_ have been noticed as far north as 72°.[380]

The vertical range of Pelagic Mollusca has received attention from Dr. Murray of the _Challenger_, Professor Agassiz of the _Blake_ and _Albatross_, and others. Agassiz appears to have established the fact that the surface fauna of the sea is limited to a comparatively narrow belt of depth, and that there is no intermediate belt of animal life between creatures which live on or near the bottom and the surface fauna. Pelagic forms sink to avoid disturbances of various kinds, to depths not much exceeding 150 to 200 fathoms, except in closed seas like the Gulf of California and the Mediterranean, where the bathymetrical range appears to be much greater.[381]

Non-Pelagic Mollusca are, from one point of view, conveniently classified according to the different _zones of depth_ at which they occur. Thus we are enabled to distinguish Mollusca of (_a_) the _littoral_, (_b_) the _laminarian_, (_c_) the _nullipore_, or _coralline_, and (_d_) the _abyssal_ zones. It must be borne in mind, however, that these zones cannot be exactly defined, and that while the littoral zone may be understood to imply the area between tide-marks, and the abyssal zone a depth of 500 fathoms and upwards, the limits between the laminarian and the coralline, and between the coralline and abyssal zones can only be fixed approximately.

The difficulty of assigning special genera or species to special ‘zones of depth’ is increased by two important facts in the phenomena of distribution. In the first place, it is found that species which occur in shallow water in northern seas often extend to very deep water in much lower latitudes. This interesting fact, which shows the importance of temperature in determining distribution, was first established by the dredgings of the _Lightning_ and _Porcupine_ off the western coasts of Europe. In the second place, a certain number of species seem equally at home in shallow and in abyssal waters, in cases where a great difference of latitude does not occur to equalise the temperature. Thus the _Challenger_ found _Venus mesodesma_ living on the beach (New Zealand) and at 1000 fath. (Tristan da Cunha); _Lima multicostata_ in ‘shallow water’ (Tonga and Port Jackson) and at 1075 fath. (Bermuda); _Scalaria acus_ from 49 to 1254 fath. (N. Atlantic); and _S. hellenica_ from 40 to 1260 fath. (Canaries). The _Lightning_ and _Porcupine_ found, or record as found,[382] _Anomia ephippium_ at 0 to 1450 fath., _Pecten groenlandicus_ at 5 to 1785 fath., _Lima subauriculata_ at 10 to 1785 fath., _Modiolaria discors_ at 0 to 1785 fath., _Crenella decussata_ at 0 to 1750 fath., _Dacrydium vitreum_ at 30 to 2750 fath., _Arca glacialis_ at 25 to 1620 fath., _Astarte compressa_ at 3 to 2000 fath., and _Scrobicularia longicallus_ at 20 to 2435 fath. _Puncturella noachina_ has been found at 20 to 1095 fath., _Natica groenlandica_ at 2 to 1290 fath., _Rissoa tenuisculpta_ at 25 to 1095 fath. In many of these cases we are assured that no appreciable difference can be detected between specimens from the two extremes of depth.

In spite, however, of these remarkable vagaries on the part of certain species, we are enabled roughly to distinguish a large number of genera as ‘shallow-water’ and ‘deep-water’ respectively, while a still larger number occupy an intermediate position. Among shallow-water genera may be named _Patella_, _Littorina_, _Nassa_, _Purpura_, _Strombus_, _Haliotis_, _Mytilus_, _Cardium_, _Solen_; while among deep-water genera are _Pleurotoma_, _Scissurella_, _Seguenzia_, _Dentalium_, _Cadulus_, _Limopsis_, _Nucula_, _Leda_, _Lima_, and _Axinus_.

Theories on the geographical distribution of marine Mollusca have been revolutionised by the discoveries of recent exploring expeditions. The principal have been those of Torell (Swedish) (1859–61) on the coasts of Greenland and Spitzbergen; of the _Lightning_ and _Porcupine_ (British) in 1868–70, in the N.E. Atlantic, off the Scotch, Irish, French, and Portuguese coasts, and in the Mediterranean; of the _Challenger_ (British), under Sir C. Wyville Thomson, in 1873–76, in which all the great ocean basins were dredged or sounded; of the _Blake_ (American), under Alexander Agassiz, in 1877–80, in the West Atlantic, Gulf of Mexico, and Caribbean Seas; of the _Travailleur_ (French) in 1880–83, off the west coasts of France, Portugal, and Morocco, Madeira, the Canaries, and the Golfe du Lion; of the _Talisman_ (French) in 1882, off the west coast of Africa from Tangier to Senegal, the Atlantic Islands, and the Sargasso Sea; of the _Albatross_ (American) in 1891, off the west coast of tropical America; of several other vessels belonging to the U.S. Fish Commission and Coast Survey, off east American shores; and of the Prince of Monaco in the _Hirondelle_ and _Princesse Alice_ at the present time, in the N. Atlantic and Mediterranean.

The general result of these explorations has been to show that the marine fauna of very deep water is much the same all the world over, and that identical species occur at points as far removed as possible from one another. The ocean floor, in fact, with its uniform similarity of temperature, food, station, and general conditions of life, contains no effectual barrier to the almost indefinite spread of species.[383] To give a few instances. The _Challenger_ dredged _Silenia Sarsii_ in 1950 fath., 1100 miles south-west of Australia, and also in 2650 fath. off the mouth of the Rio de la Plata; _Semele profundorum_ in 1125 fath. near the Canaries, and in 2900 fath. mid N. Pacific; _Verticordia deshayesiana_ in 155 fath. near Cape York, and in 350 fath. off Pernambuco; _Arca pteroessa_ in 2050 fath. mid N. Pacific, in 1000–1675 fath. west of the Azores, and in 390 fath. off the West Indies; _Arca corpulenta_ in 1400 fath. off N.E. Australia, in 2425 fath. mid-Pacific, and in 1375 fath. near Juan Fernandez; _Lima goliath_ in 775 fath. off S. Japan, and in 245 fath. off S. Patagonia; _Pleurotoma engonia_ in 700 fath. north-east of New Zealand, and in 345 fath. off Inoshima. A surprising range was occasionally found even in shallow-water species; thus _Petricola lapicida_ was discovered by the same expedition in the West Indies and N. Australia, _Cardita calyculata_ off Teneriffe and in Bass Strait, _Arca imbricata_ off Cape York and in the West Indies, _Modiolaria cuneata_ at Port Jackson and Cape of Good Hope, _Lima squamosa_ at Teneriffe and the Philippines. In these latter cases it is not improbable that the species lives in deep water as well, from which it has not yet been dredged.

It follows from these considerations that any attempt to classify marine Mollusca under Regions and Provinces can only apply to Mollusca which occur at moderate depths. The most important factor in the environment, as determining distribution, is the _temperature of the water_, which is probably to be regarded as affecting not so much the adult Mollusca as their ova; for the adult might possibly support life under conditions in which the ova would perish. It appears that a sudden change of temperature is the most effective barrier to distribution,[384] and may bring the range of a species to an almost instantaneous stop, while a very gradual change will allow it to extend its range very widely.

It has been usual to classify marine Mollusca from moderate depths under the following regions and sub-regions:--

Regions Sub-regions

{1. Arctic. {2. Boreal. =A.= =Atlantic= and {3. Celtic. =Circumpolar= {4. Lusitanian. {5. West African. {6. South African.

=B.= =Indo-Pacific= {1. Indo-Pacific. {2. Japanese.

=C.= =Australian= {1. Australian. {2. Neozealanian.

{1. Aleutian. {2. Californian. {3. Panamic. =D.= =American= {4. Peruvian. {5. Magellanic. {6. Argentinian. {7. Caribbean. {8. Transatlantic.

=A. The Atlantic Region=

includes the whole to the eastern shores of the Atlantic, from the extreme north of the Cape of Good Hope, together with the circumpolar seas, which may be regarded as roughly bounded by the Aleutian Islands and the coast of Newfoundland.

(1) _The Arctic Sub-region_ includes the circumpolar seas, and is bounded in the N. Pacific by a line drawn between Cape Avinoff in Alaska, and Cape Lopatka in Kamchatka, so as to exclude the Aleutian Islands. On the western shores of the Atlantic the cold Labrador current brings it as far south as the coast of Newfoundland, but on the eastern shores the influence of the Gulf Stream has the contrary effect, so that the North Cape may be taken as its southern limit.

The principal genera (many species of which are common to the whole sub-region) are _Volutomitra_, _Buccinum_, _Buccinopsis_, _Neptunea_, _Trophon_, _Bela_, _Admete_, _Velutina_, _Trichotropis_, _Lacuna_, _Margarita_, _Philine_, _Pecten_, _Leda_, _Yoldia_, _Astarte_, and _Mya_. The shells are generally unicoloured, and of a dead white or rather sombre tint.

(2) _The Boreal Sub-region_ may be subdivided into two provinces, the European and the American. The former includes the entire coast-line of Norway, the Färoe Islands, and Iceland (except perhaps the northern coast), and possibly the Shetland Islands; the latter the American coasts from the Gulf of St. Lawrence to Cape Cod (lat. 42°). Thus the Boreal American province does not extend nearly so far south as the Boreal European, the reason being that on the American coasts the cold Labrador current, which hugs the land, bars back the advance of southern genera, but allows Boreal genera to spread southwards, while on the European side the warmer conditions produced by the Gulf Stream keep the Boreal species back, and allow more southern forms to spread northwards.

Many of the Boreal species occur on both sides of the Atlantic, and thus support the theory of a more continuous fringe of continental land once existing along the north of the Atlantic. Among the prominent genera, besides several of those mentioned under the Arctic Sub-region, are _Purpura_, _Chenopus_, _Littorina_, _Gibbula_, _Natica_, _Patella_, _Tectura_, _Chiton_, _Doris_, _Aeolis_, _Tellina_, _Thracia_.

(3) _The Celtic Sub-region_ includes the British Islands (excepting perhaps the Shetland Islands), the coasts of the North Sea and the Baltic, with N. France to Cape Ushant. The absence of any cold or warm current exerting direct influence upon the coast-line of this sub-region causes a very gradual change in the conditions of life as we move either southward or northward. The fauna of the British seas contains a decided mixture of northern and southern forms. The following are among the common Boreal species which attain their southward range on our coasts: _Tectura testudinalis_ Müll. (to Dublin Bay and Scarborough), _Trichotropis borealis_ Brod. (to the Dogger Bank), _Margarita helicina_ Fabr. (to Yorkshire and Dublin Bay), _M. groenlandica_ Chem. (western Scotland), _Natica montacuti_ Forb. (to Cornwall), _Trophon truncatus_ Str. (to Tenby), _Chiton marmoreus_ Fabr. (to Dublin Bay and Scarborough). _Buccinum undatum_ and _Littorina littorea_ become very scarce on our extreme south-western coasts. Among Lusitanian species which reach our coasts are _Gibbula magus_ L. (to Orkney and Shetland Islands), _Phasianella pullus_ L. (to Caithness), _Galerus chinensis_ L. (to Milford Haven), _Galeomma Turtoni_ Turt. (to Weymouth), _Cardium aculeatum_ L. (to Isle of Man), _Solen vagina_ L. (to north Ireland).

It appears from the Mollusca of our Crag formations that at the time of their deposition the temperature of our seas must have been considerably warmer than it is now. Thus in the Crag we find many species and even genera (_e.g._ _Mitra_, _Fossarus_, _Triton_, _Vermetus_, _Ringicula_, _Chama_) which now occur no farther north than the southern coasts of the Channel, the west of France, and the Mediterranean.

The Baltic, a sea specially liable to violent changes of temperature, with a large admixture of fresh water at its eastern end, appears to possess only about 65 species in all. More than 50 genera occurring on the western coasts of Denmark do not enter the Sound. In the eastern portion of the Baltic marine and fresh-water species live together (p. 12).

(4) _The Lusitanian Sub-region_ extends from Cape Ushant in the north to Cape Juby (lat. 28°) in the south, and includes the whole of the Mediterranean, as well as the Azores, Canaries, and Madeira groups.

The English Channel acts as an effectual barrier to the northward extension of many species; as many as 81 species which occur in western France do not reach British coasts (P. Fischer). At the same time, the western coasts of France are rather intermediate between the two sub-regions than distinctly Lusitanian, for between 50 and 60 Mediterranean genera do not occur on those coasts.

The Mediterranean itself is exceedingly rich in species, about 1200 in all (including deep-water species) being known. A certain number of these belong to tropical genera which here find their northern limit, _e.g._ _Fasciolaria_, _Cancellaria_, _Sigaretus_, _Siliquaria_, _Chama_, _Spondylus_. Here too occur _Carinaria_, _Lobiger_, _Oxynoe_, _Pedicularia_, _Cypraea_, _Marginella_, _Mitra_, _Dolium_, _Cassis_, _Cassidaria_, _Pisania_, _Euthria_, _Vermetus_, _Argonauta_, and many others. A few Celtic and even Boreal species, which occur on the western coasts of Morocco, do not enter the Mediterranean. Among these are _Purpura lapillus_, _Helcion pellucidum_, and _Tellina balthica_. _Halia_, a rare West African genus akin to _Pleurotoma_, is found in Cadiz Bay, and the West African _Cymbium_ occurs on the Spanish coasts as far as Malaga.

The Black Sea, whose northern and western coasts are exceedingly cold, is comparatively poor in species. The Sea of Azof is chiefly characterised by forms of _Cardium_.

(5) _The West African Sub-region_ extends from Cape Juby to a point probably not very far south of lat. 30° S., the cold current which sweeps up from the Pole probably limiting the southward extension of tropical species on this side of Africa, while the warm Mozambique current on the eastern side permits the spread of many Indo-Pacific species almost as far south as the Cape. Owing to its extreme unhealthiness, and the absence of harbours, the sub-region is very little known.

The principal genera are _Cymbium_, _Pleurotoma_, _Marginella_, _Terebra_, _Mitra_, _Agaronia_, _Murex_, _Cancellaria_, _Purpura_, _Pseudoliva_, _Natica_, _Tellina_, _Lucina_, _Tugonia_, _Schizodesma_, and _Arca_. Studer has enumerated as many as 55 species common to West Africa and the opposite American shores. The north and south equatorial currents, which circulate in this part of the Atlantic, probably transport the larvae from one coast to the other. _Purpura coronata_ Lam., a characteristic West African species, is represented by a well-marked variety in Demerara.

The Mollusca of St. Helena (178 known species) most resemble those of the West Indies, 50 per cent being common, while 30 per cent are common to the Mediterranean. From Ascension Island only 33 species are known, which in their general relations resemble those of St. Helena.[385]

(6) _The South African Sub-region_ extends along the coast from about lat. 30° on the west, to about East London on the east. Mr. G. B. Sowerby enumerates 740 species from ‘South Africa,’ but includes in this list Natal species, which more properly belong to the Indo-Pacific fauna. Of these 740, 323 are peculiar, while 67 also occur in European seas, some being familiar on our own shores. It is remarkable to find in a sub-region separated from ourselves by the whole width of the tropics, such well-known forms as _Mangilia costata_ Don., _M. septangularis_ Mont., _Cylichna cylindracea_ Penn., _Pholas dactylus_ L., _Solen marginatus_ Pult., _Cultellus pellucidus_ Penn., _Ceratisolen legumen_ L., _Lutraria oblonga_ Chem., _Tellina fabula_ Gmel., _T. tenuis_ Da C., _Modiolaria discors_ L., and many others.

The leading genera are _Euthria_, _Triton_, _Cominella_, _Bullia_, _Nassa_, _Cypraeovula_, _Oxystele_, _Fissurella_, _Fissurellidaea_, _Patella_, and _Chiton_.

The Mollusca of Kerguelen Island and the Marion and Crozets groups show relationship partly with South America, partly with the Cape, and

## partly with South Australia and New Zealand, thus showing some trace

of a circumpolar antarctic fauna corresponding to, but not nearly so well marked as that of the circumpolar arctic sub-region. Among the remarkable forms discovered off Kerguelen are _Neobuccinum_ and a sub-genus of _Struthiolaria_ (_Perissodonta_).

=B. The Indo-Pacific Region=

includes the whole of the coast-line of the Indian and western Pacific oceans, from about East London in South Africa to the north of Niphon (lat. 42°) in Japan, with the Red Sea and Persian Gulf, the whole of the Indo-Malay Archipelago, Polynesia to the Sandwich Islands in the north-east, and Easter Island in the south-east, and Australia to Swan River in the west, and to Sandy Cape and Lord Howe’s Island in the east. It is especially the region of coral reefs, which furnish so favourite a home of the Mollusca, and which are entirely absent from the Atlantic Region.

(1) _The Indo-Pacific Sub-region proper_ (which includes the whole of this region except that part defined below as the Japanese Sub-region) is by far the richest in the world. The marine Mollusca of the Philippines alone (in some respects the nucleus of the whole region) have been estimated at between 5000 and 6000 species, and Jousseaume estimates Red Sea species at about 1000. Some prominent genera are very rich in species. Garrett enumerates from Polynesia 81 species of _Conus_, 60 of which occur on the Viti Is., 21 on the Sandwich Is., and only 14 on the Marquesas, where coral reefs are almost absent; 82 species of _Cypraea_, Viti Is. 44, Sandwich Is. 31, Marquesas only 13; 167 species of _Mitra_ (besides 29 recorded by others), Viti Is. 120, Sandwich Is. 36, Marquesas 7. Of 50 existing species of _Strombus_, 39 occur in this region, and 10 out of 11 _Eburna_.

The following important genera are quite peculiar to the region: _Nautilus_, several forms of Purpuridae, _e.g._ _Rapana_, _Magilus_, _Rapa_, _Melapium_, and _Ricinula_; _Tudicla_, several forms of Strombidae, _e.g._ _Rostellaria_, _Terebellum_, _Pteroceras_, and _Rimella_; _Cithara_, _Melo_, _Neritopsis_, _Stomatia_, _Malleus_, _Vulsella_, _Cucullaea_, _Tridacna_, _Hippopus_, _Libitina_, _Glaucomya_, _Anatina_, _Aspergillum_, and many others.

The number of species common to the Red Sea and Mediterranean is exceedingly small, some authorities even denying the existence of a single common species. The present author, from an examination of the shells dredged by MacAndrew at Suez, regarded 17 species as common, and Mr. E. A. Smith has confirmed this view with regard to 8 of the species in question.[386] The Mollusca occurring in Post-pliocene beds at Suez show that Mediterranean species lived there in comparatively recent geological times.

The opening of the Suez Canal appears to have already induced several species to start on their travels from the Mediterranean to the Red Sea and _vice versâ_. Two Red Sea species (_Mactra olorina_ Phil., _Mytilus variabilis_ Kr.) had in 1882 established themselves at Port Said, while two Mediterranean species (_Pholas dactylus_ L., _Solen vagina_ L.) had reached Ismailia.[387]

(2) _The Japanese Sub-region_ consists of the Japanese Islands to Niphon, together with Corea and a stretch of adjacent mainland coast of unknown extent. The warm Kuro Siwo current, sweeping up between Luzon and Formosa, permits tropical species to extend much farther north than on the opposite shores of America, where a cold polar current keeps them back. A certain number of species, however, are common to the two shores of the Pacific, and a few circumpolar species occurring on our own coasts reach Japan, _e.g.__Trophon clathratus_, _Puncturella noachina_, _Mya arenaria_, _Modiola modiolus_, _Lasaea rubra_, and _Nucula tenuis_.

Among the characteristic genera are _Fusus_, _Siphonalia_, _Columbarium_, _Hemifusus_, _Rapana_, _Chlorostoma_, _Pleurotomaria_, _Haliotis_, and _Cyclina_.

=C. The Australian Region=

includes the Australian coast-line from about Swan R.[388] (lat. 32° S.) to Sandy Cape (lat. 25° S.), Tasmania, New Zealand, and the adjacent islands (except Lord Howe’s I.).

(1) _The Australian Sub-region proper_ (which consists of the whole of the region excepting New Zealand and the adjacent islands) is determined by the influence of the Antarctic Drift, which washes the whole of the southern coasts of Australia, and runs strongly northward between Australia and New Zealand. The E. Australian warm current from the north is checked at Sandy Cape by this cold current, and flows off to New Zealand, the western shores of which island are consequently much warmer than the eastern. On the western coast of Australia the Antarctic Drift has less force, and tropical genera accordingly range some 7 degrees farther south on the western than on the eastern coasts.

The characteristic genera are _Voluta_ (of which half the known species occur on Australian coasts[389]), _Cominella_, _Siphonalia_, _Struthiolaria_, _Risella_, _Phasianella_, a number of genera belonging to the Trochidae, _e.g._ _Liotia_, _Clanculus_, _Euchelus_, _Thalotia_, _Elenchus_, _Trochocochlea_, _Zizyphinus_, _Bankivia_, _Trigonia_, _Myodora_, _Myochama_, _Solenomya_, _Ephippodonta_, _Anapa_, _Mylitta_, _Mesodesma_, and _Chamostrea_. _Trigonia_, originally discovered as a recent form in Sydney Harbour (p. 65), is not peculiar to that locality, occurring also off Cape York, West Australia, and Tasmania.

(2) _The Neozealanian Sub-region_ includes New Zealand, with the outlying islands (Chatham, Auckland, and Campbell Is.).

As many as 455 species (Cephalopoda, 8; Gasteropoda, 311; Scaphopoda, 2; Pelecypoda, 134) have been enumerated by Professor F. W. Hutton as occurring within the sub-region, of which only 64 are found elsewhere, the proportion of peculiar species being thus nearly 86 per cent. New Zealand therefore is, in its marine, no less than its land Mollusca, greatly isolated.

The characteristic genera are _Anthora_, _Cryptoconchus_, and _Vanganella_, which appear to be quite peculiar, _Trophon_, _Cominella_, _Euthria_, most of the Trochidae also characteristic of S. Australia, _Haliotis_, _Patella_, _Taria_, _Mesodesma_, _Mylitta_, _Zenatia_, _Standella_, and _Myodora_.

=D. The American Region=

includes the entire coasts of North and South America with the adjacent islands, south of Cape Avinoff on the western, and south of Cape Cod on the eastern coast, the portions north of these points belonging to the Arctic Sub-region.

(1) _The Aleutian Sub-region_ consists of the islands of Yesso and Saghalien, with the adjacent shores of the Sea of Okhotsk to Cape Lopatka, the Aleutian Is., and the west American coast from about Cape Avinoff (lat. 60° N.) to St. Jean de Fuca Straits.

A certain number of species, probably of arctic origin, are common with British and also with East American shores, the former being the more numerous. Species as familiar to us as _Lacuna divaricata_ Fabr., _Trichotropis borealis_ Brod., _Pholas crispata_ L., _Mya truncata_ L., _M. arenaria_ L., _Mytilus edulis_ L., and _Modiolaria nigra_ Gray, occur. The more characteristic genera are _Chrysodomus_, _Volutharpa_, _Buccinum_, _Tectura_, _Scurria_, _Chiton_, _Cryptochiton_ (_Cr. Stelleri_ Midd. is by far the largest known of the Chitonidae, 6 inches long), _Tellina_ and _Pecten_.

(2) _The Californian Sub-region_ extends from St. Jean de Fuca Straits (lat. 48° N.) to Cape St. Lucas, the Gulf of California belonging to the Panamic sub-region. The northern polar current, which washes the shores of this sub-region throughout their whole extent, prolongs the southward range of the more northern genera, and keeps back those more markedly tropical, the latter, however, creeping northward in the warmer waters of the Gulf of California. Some authorities subdivide this immense stretch of coast-line, as characterised by sub-temperate, temperate, and sub-tropical genera, into the Oregonian, Californian, and Lower Californian provinces.

The characteristic genera are--in the north, _Argobuccinum_, _Zizyphinus_, _Chlorostoma_, _Tectura_, _Scurria_, _Chiton_ (_Katharina_, _Mopalia_, _Tonicia_), _Cryptochiton_, _Placunanomia_, and _Mytilimeria_; in the centre, _Purpura_, _Monoceros_, _Amphissa_, _Norrisia_, _Platyodon_, _Tapes_, and _Macoma_; and, towards the south, _Olivella_, _Chorus_, _Macron_, _Pseudoliva_, _Trivia_, and _Haliotis_.

(3) _The Panamic Sub-region_ extends from the head of the Gulf of California to Payta in Peru (lat. 5° S.). It is exceedingly rich in species, about 1500 having been described. The Mollusca are entirely distinct from those of the Indo-Pacific Region, which, although extending from Natal to the Sandwich Islands, are unable to pass the enormous extent of sea which separates the nearest Polynesian island from the American coast.

On the two sides of the isthmus of Panama there occur certain pairs of species, which, while specifically distant, are evidently closely related to one another. Amongst these are, on the Panamic side, _Purpura speciosa_, _Cypraea cervinetta_, _Cassis abbreviata_, _Natica Chemnitzii_, and _Strombus gracilior_, corresponding to _Purpura deltoidea_, _Cypraea exanthema_, _Cassis inflata_, _Natica maroccana_, and _Strombus pugilis_, on the Caribbean. It is reasonable to conclude that these “analogous species” are descendants of a stock which was common to both seas when the isthmus was open (probably not later than Miocene times), and which have, since the closing of the isthmus, become modified, some species considerably more than others.

Among the characteristic genera (compare p. 3) are _Conus_, _Pleurotoma_, _Terebra_, _Murex_, _Purpura_, _Oliva_, _Northia_, _Cantharus_, _Columbella_, _Anachis_, _Cypraea_, _Strombus_, _Cerithium_, _Coecum_, _Crepidula_, _Crucibulum_, _Vitrinella_; _Tellina_, _Semele_, _Tellidora_; and _Arca_.

(4) _The Peruvian Sub-region_ extends from Payta in Peru to about the latitude of Conception in S. Chili (37° S.), being checked from further extension southward by the cold Humboldt current, whose force is distinctly felt as far north as Callao. This cold current thus produces the same results as the similar current which impinges on S. Africa, but has even more effect in decisively separating the fauna on the two sides of the great peninsula, scarcely a single species being common to the western and eastern coasts of S. America. The characteristics of the coast-lines themselves contribute to this result. The Chilian coast is rocky, and descends abruptly to a great depth, while that of Patagonia and Argentina is sandy and very shallow to a great distance from land.

The characteristic genera are _Cancellaria_, _Columbella_, _Monoceros_, _Concholepas_, _Trochita_, _Fissurella_, _Chiton_; _Ceronia_, _Malletia_, and _Cumingia_. Some of the Californian genera, absent or poorly represented in the Panamic Sub-region, reappear in Chili, _e.g._ _Scurria_, _Tectura_, and _Chlorostoma_.

(5) _The Magellanic Sub-region_ includes the coast-line and adjacent islands (with the Falklands) from Conception in S. Chili to about Port Melo in Eastern Patagonia (lat. 45° S.).

The principal genera (many of which find a habitat on the gigantic Macrocystis which grows on every rock at low water) are _Euthria_, _Voluta_ (6 species, one, _V. magellanica_, the largest known) _Monoceros_, _Photinula_, _Patella_, _Chiton_; _Modiolarca_, _Malletia_, and _Mulinia_. Several genera characteristic of the Boreal and Arctic sub-regions recur, _e.g._ _Trophon_, _Admete_, _Margarita_, _Puncturella_, _Cyamium_, and _Astarte_.

(6) _The Argentinian[390] Sub-region_ extends from about Cape Melo in Patagonia to the neighbourhood of S. Caterina I. in South Brazil (lat. 28° S.). The sub-region stands in the same relation to the Magellanic, on the east coast, as the Peruvian sub-region on the west, but, owing to the influence of the warm Brazil current, which overpowers the colder water of the Falkland branch of the Cape Horn current, it reaches a point much farther south.

The Mollusca are not well known. The prevailing genera appear to be _Oliva_, _Olivancillaria_, _Voluta_, _Bullia_, _Crepidula_; _Periploma_, and _Lyonsia_.

(7) _The Caribbean Sub-region_ extends from S. Caterina I. in the south to Florida in the north, and includes the shores of the Gulf of Mexico and the whole of the West Indies. The influence of the warm Brazil current (a branch of the South Equatorial) carries the range of the purely tropical species to a point much farther south than is reached by the tropical species on the west coast.

The sub-region is very rich in species, especially on the coral reefs of the Bahamas and N. Cuba, but the exceedingly small tide-fall makes shore collecting somewhat difficult beyond a certain point. The leading genera are _Murex_, _Purpura_, _Melongena_, _Latirus_, _Marginella_, _Strombus_, _Triton_, _Cerithium_, _Littorina_, _Nerita_, _Scalaria_; _Tellina_, _Strigilla_, _Lucina_, and _Venus_. _Pleurotomaria_, a genus long regarded as extinct, has been dredged alive off Tobago.

As compared with the tropical fauna of the Old World, that of the New World is poor in peculiar genera (compare p. 368). The relations of this sub-region to the West African and the Panamic have been already dealt with (pp. 367 and 372).

(8) _The Transatlantic Sub-region_ extends from Florida to Cape Cod (see p. 364). In the north the limits of the sub-region are distinctly marked, in the south Caribbean species intermingle. Gould and Binney, in their _Invertebrata of Massachusetts_, enumerate 275 species (Cephalopoda, 6; Gasteropoda, 159; Scaphopoda, 2; Pelecypoda, 108), of which 59 (Gasteropoda, 37; Pelecypoda, 22) are British.

Among the characteristic genera are _Urosalpinx_, _Eupleura_, _Fulgur_, _Ptychatractus_, _Nassa_, _Crepidula_; _Solenomya_, _Mactra_, _Cypricia_, _Raëta_, _Astarte_, and _Yoldia_. Our common _Littorina littorea_ appears to have been introduced into Nova Scotian waters in about 1857, no previous trace of it occurring either in literature or shell-heaps. Since then it has spread rapidly into the Gulf of St. Lawrence, and also as far south as Newhaven, and is said to be driving out the indigenous _L. palliata_ from New England shores.[391] The debt has been repaid by the introduction into British waters of the American clam (_Venus mercenaria_ L.), which, according to the _Manchester City News_ of 23rd March 1889, was first observed in the Humber in 1864, and has steadily increased up to the present time, when it bids fair to compete, in those waters, with the familiar _Cardium edule_.

=Characteristics of Abyssal Mollusca.=--Large shells appear to be rare in the great ocean depths, and are usually very fragile; even moderately-sized specimens are far from common. The only group in which species occur larger than the usual size is the Nudibranchs, which are represented by at least one form larger than an orange.

It would seem that abyssal molluscs are much less active and energetic than their brethren on the shores. This view is favoured by the looseness of their tissues, which seem ill adapted for prompt and vigorous action. The tenacious character of the mud on the ocean floor must make rapid motion very difficult. The shell itself is usually fragile and delicate, the upper layers of arragonite being thin as compared with shallow-water species, which makes the nacreous layer, when present, appear unusually conspicuous; in many cases the surface is characterised by a peculiar iridescence or sheen. The colour in the shell of deep-sea Mollusca is never very pronounced, and is often absent altogether. Light pink and salmon, pale yellow and brown, are not uncommon. If the colour is in pattern, it is usually in the form of necklaces of spots, which sometimes coalesce into bands. With regard to sculpture, stout knobs and powerfully buttressed varices, such as occur in the tropical _Murex_ and _Purpura_, are not found in deep-sea species. But the ornamentation is frequently elaborate, and the sculpture rich and varied. There is an especial tendency towards strings of bead-like knobs, revolving striae, and delicate transverse waves, the sculpture being in many cases of a character which tends to strengthen the structure of the shell, like the ridges in corrugated iron.

A remarkable feature in some deep-sea Mollusca is their singular resemblance, in shape, and particularly in the possession of a strong green periostracum, to some of our common fresh-water species. According to Dr. Dall, the cause of this phenomenon is the same in both cases. The fresh-water Mollusca secrete a strong periostracum, in order to protect the shell against the corrosive influence of the carbonic acid gas with which the water is surcharged. The shells of deep-sea Mollusca, living, as they do, in water probably undisturbed by currents of any kind, have to protect themselves against the same eroding influence, and do so in the same way.[392]

Mollusca which live exclusively on algae and other forms of plant life are almost entirely wanting in the great depths, where vegetation is probably unknown. The struggle for existence must be much less keen than in the thickly populated shallows, where vicissitudes of every kind occur. The absence of rapid motion of water must obliterate many of those mechanical effects which tend to produce modifying influences upon the animals affected. In the absence of circumstances tending to cause variation, in the unbroken monotony of their surroundings, species must, one would think, preserve a marked uniformity over an exceedingly wide area of range.

Vegetable food being wanting, those genera which in shallower waters never taste flesh, are compelled to become carnivorous. Characteristic of the great depths are very remarkable forms of Trochidae, in whose stomachs have been found the remains of Corallines and Foraminifera. According to Dr. Dall, the results of this diet show themselves in the greatly increased space occupied by the intestine, in the diminution, as regards size, of the masticatory organs, the teeth and jaws, and also in the prolongation of the anal end of the intestine into a free tube, which carries away the excreta in such a way that they do not foul the water taken into the gills. The amount of nutriment contained in the bodies of dead Foraminifera is so small that a comparatively large quantity must be swallowed to keep the vital energies active, and therefore the amount evacuated must be proportionately larger also. The abyssal Trochidae, then, and many other genera, sustain themselves by feeding on the ‘rain’ of dead animal matter which falls upon the ocean floor, not so much hunting their prey as opening their mouths and eating whatever happens to fall into them. Genera which are normally carnivorous would appear to do the same. The Pleurotomidae, for instance, are a family markedly characteristic of very deep water. Representatives of the genus which occur in shallower water are known to secure their prey while in the living state. But, according to Dr. Dall, a singularly small proportion of deep-sea Mollusca, as compared with those from the littoral region, show signs of having been drilled or attacked by other Mollusca. This could hardly be the case if the Pleurotomidae retained their predatory habits, since they are more numerous in the great depths than any six other families taken together. It has already been mentioned (p. 186) that a large proportion of deep-sea Mollusca are perfectly blind.

Amongst other remarkable forms from the great depths may be mentioned _Pleurotomaria_, with its singular anal slit (Fig. 269, p. 407) extending in some cases half way round the last whorl. Three or four species of this genus, so characteristic of almost all fossiliferous strata down to the Cambrian, have been obtained in very limited numbers off the West Indies and Japan. Dentaliidae, especially the sub-genus _Cadulus_, find a congenial home in the slimy ocean mud. One of the greatest molluscan treasures procured by the _Challenger_ was _Guivillea alabastrina_ Wats., a magnificent Volute as white as alabaster, 6½ inches long, which was dredged from 1600 fath. in the South Atlantic, between Marion Island and the Crozets. Another very curious form, belonging to the same family, is _Provocator pulcher_ Wats., a shell about half the size of _Guivillea_, of stouter proportions, and with an angulated and patulous mouth. This shell was dredged by the _Challenger_ in comparatively shallow water (105–150 fath.) off Kerguelen Island. Among the Trochidae are the fine new genera _Basilissa_, _Bembix_, and _Gaza_. The exploring voyages of the American surveying steamer _Blake_, in the Gulf of Mexico and the Caribbean Sea, have given us the remarkable new forms _Benthobia_ (possibly akin to _Admete_), _Mesorhytis_ (a sub-genus of _Fasciolaria_ hitherto only known from the Cretaceous of North America), and _Benthodolium_ (possibly = _Oocorys_), a genus akin to _Cassis_.

In his report on the Pelecypoda obtained by the _Challenger_, Mr. E. A. Smith remarks that as a rule “very deep-water ‘benthal’ species certainly have a tendency to be without colour and of thin structure, facts no doubt resulting from the absence of light, the difficulty of secreting lime, the scarcity of food, and other unfavourable conditions of existence.” At the same time, he notices that most of the species obtained belong to genera which, even when occurring in shallow water, are thin and colourless, _e.g._ _Neaera_, _Lima_, _Cryptodon_, _Abra_, _Verticordia_, etc. Deep-water species of such genera as have a decided periostracum (_Malletia_, _Limopsis_, _Leda_, _Nucula_, _Arca_) retain it with little if any modification. The deep-water Pelecypoda of the Atlantic and Pacific Oceans present no special features of interest. The species are few in number, and the genera are not remarkable either for novelty or peculiarity of form.

The greatest depth at which Pelecypoda have been obtained is 2900 fath. mid North Pacific (_Callocardia pacifica_ Sm., _Abra profundorum_ Sm.); the greatest depth at which Gasteropoda have been obtained is 2650 fath. South Atlantic (_Stylifer brychius_ Wats.), both by the _Challenger_. The deepest _Challenger_ Nudibranch came from 2425 fath., and the deepest _Chiton_ from 2300 fath. The greatest depth ever dredged is 4575 fath. off the east coast of Japan.

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