Chapter X (1)

GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOLLUSCA--THE
PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN REGIONS

The Mollusca afford specially valuable evidence on problems of geographical distribution. This fact is largely due to their extreme susceptibility to any change in the conditions of life. Genera which are accustomed to live in a certain temperature and on certain food, cannot sustain life if the temperature falls or rises beyond certain limits, or if the required food be not forthcoming. There is therefore a marked contrast between the Mollusca of the tropics and of the temperate zones, while different regions in the same latitude, whether within or without the tropics, often show great diversity in their fauna. Every region is thus _characterised_ by its Mollusca. The Mollusca, for instance, of Australia or of South Africa characterise those countries quite as much as do the kangaroo and the emu, the hartebeest and the ostrich; there is nothing like them anywhere else in the world. In the Greater Antilles the Mollusca stand out beyond all other forms of life as characteristic of the islands as a whole, and of each separate island in particular.

The geographical distribution of the land and fresh-water Mollusca must be considered quite apart from that of the marine Mollusca. The sea offers no such serious barriers to the spread of the latter as the land does to the spread of the former. If we were to journey to the Azores, and turn our attention to the land-snails, we should find them almost wholly peculiar, while amongst the sea-shells we should recognise many as occurring also on our southern coasts, and few that were different from those of the Mediterranean. The marine Mollusca of the Sandwich Islands, in spite of the enormous intervening distance, are not very different from those of Natal, but the land Mollusca of the two countries are as widely different as is possible to imagine.

Land Mollusca are, as has been remarked, fettered to the soil. Quadrupeds, birds, fishes, and reptiles are provided with organs of motion which enable them to overpass barriers of various kinds. Even plants, although themselves incapable of motion, may be conveyed in every direction by means of seeds, which are either wafted by the wind or adhere to the skin of animals. But the Mollusca have no such regular means of transport, and are, in a large number of instances, limited to districts of a certain character of soil, or producing certain kinds of vegetation.

The localisation, both of genera and species, occurs all over the world. The genus _Achatinella_, which is peculiar to the Sandwich Islands, is found there in a profusion of species. It lives in the mountain valleys which radiate from the central ridge of each island, and each valley is characterised by its own peculiar set of species. The great carnivorous _Glandina_ is restricted to Central America and the adjacent parts of the two continents, with one or two species in Southern Europe. _Bulimus_ proper is restricted to South America; _Achatina_ to Africa south of the Sahara; _Tornatellina_ to the Pacific Islands; _Cochlostyla_ to the Philippines; _Cylindrella_ and _Bulimulus_ are peculiar to the New World; _Buliminus_, _Nanina_, _Scarabus_, and _Cassidula_ to the Old.

Extreme cases of this restriction of habitat sometimes occur. Thus _Limnaea involuta_ is found only in a single small mountain tarn in Ireland; _Clausilia scalaris_ along a narrow strip of limestone in Malta; _Strophia nana_ is confined to a few square rods on an island that is itself a mere dot in the Caribbean Sea; the genus _Camptonyx_ occurs only in the neighbourhood of Mt. Girnar, in Gujerat; and _Lantzia_ in moss on the top of a mountain in Bourbon.

Attempts to colonise snails in strange localities have usually resulted in failure, especially when the attempt has involved serious changes of environment. The common _Cochlicella acuta_ of our coasts resists all endeavours to establish it beyond a certain distance from the sea. Snails brought from the Riviera and placed under almost similar conditions of climate on our own southern coasts have lived for a while, but have very rarely taken permanent root. Mr. H. W. Kew[354] has collected a good many of these attempts to acclimatise species, the general success of which seems to depend almost entirely on a restoration of the old conditions of life.

At the same time there are certain species which exhibit a curiously opposite tendency, and which seem capable of flourishing in almost any part of the world, and under the most varied surroundings. Our own common garden snail (_Helix aspersa_) is a striking instance of this adaptability to new conditions. It has been established, by art or by accident, in Nova Scotia, Maine, South Carolina, New Orleans, California, Mexico city, Cuba, Hayti, Cayenne, Brazil, Valparaiso, Cape Town, the Azores, St. Helena, Mauritius, Loyalty Islands, and Australia. The great _Achatina fulica_ of East Africa has been established first in Mauritius, and from thence has been carried to the Seychelles and Calcutta. _Helix lactea_, a common Mediterranean species, has been carried to Teneriffe and Montevideo; _Helix similaris_, whose fatherland is Eastern Asia, has been transported to Mauritius, Bourbon, West Africa, West Indies, Brazil, and Australia; _Ennea bicolor_ (Eastern Asia) to India, Bourbon, Mauritius, West Indies; _Stenogyra decollata_ (Mediterranean basin) to South Carolina; _S. Goodallii_ (West Indies) to British pineries; _Helix Hortensis_ to New Jersey. Seven common English species (_Limax gagates_, _Hyalinia cellaria_, _H. alliaria_, _Helix aspersa_, _H. pulchella_, _Pupa umbilicata_) have become naturalised in St. Helena,[355] and as many as nineteen in Australia.[356]

Cases of artificial transport of this kind are readily detected; they follow the lines of trade. The snails themselves or their ova have been accidentally enclosed with plants or mould, or have adhered to packing-cases, or to hay and grass used in packing. Thus they constitute no disturbance to the general rule of the persistent localisation of species and genera, and there is little fear that the evidence which the geographical distribution of the Mollusca brings to bear upon the general problems of distribution will be confused by any intermixture of fauna naturally distinct.

_Land Mollusca: Barriers to Dispersal._--The chief natural barriers to dispersal are extremes of temperature, the sea, mountain ranges, and deserts. Rivers, however large, seem of little effect in checking dispersal. There is no appreciable difference between the land Mollusca north and south of the Ganges, or north and south of the Amazon. Living snails, or their ova, are no doubt transported from one bank to another on floating débris of various kinds. The barrier offered by the sea is obvious, and at first sight appears insurmountable; but the facts with regard to oceanic groups of islands like the Azores and Canaries (see p. 297) show that even a stretch of salt water many hundred miles in breadth may be ineffectual in preventing the dispersal of Mollusca.

Mountain ranges, provided they are too high to be scaled, and too long to be turned in flank, offer a far more effective barrier than the sea. Every thousand feet upward means a fall of so many degrees in the mean temperature, and a change, more or less marked, in the character of the vegetation. There is generally, too, a considerable difference in the nature of the climate on the two sides of a great mountain range, one side being often arid and cold, the other rainy and warm. The combined effect of these influences is, as a rule, decisive against the dispersal of Mollusca. Thus the Helices of California are almost entirely peculiar; one or two intruders from states farther east have succeeded in threading their way through the deep valleys into the Pacific provinces, but not a single genuine Californian species has been able to scale the heights of the Cascade Mountains. The land Mollusca of India are numbered by hundreds; not one penetrates north of the Himalayas. According to Mr. Nevill,[357] the change from the Indo-Malayan to the so-called European molluscan fauna at the northern watershed of the Kashmir valley is most abrupt and distinct; in two days’ march northward, every species is different. Ranges of inferior altitude, such as the Pyrenees, the Carpathians, or the Alleghanies, may be turned in flank as well as scaled, and we find no such marked contrast between the Mollusca on their opposite sides.

The most effective barrier of all, however, is a desert. Its scorching heat, combined with the absence of water and of vegetable life, check dispersal as nothing else can. The distribution of the Mollusca of the Palaearctic Region is an excellent instance of this. Their southern limit is the great desert which stretches, with scarcely a break, from the west coast of Africa to the extreme east coast of Asia. The Mediterranean offers no effectual barrier; shells of southern Europe are found in profusion in Morocco, Tunis, and Egypt, while all through Siberia to the extreme of Kamschatka the same types, and even the same species, of Mollusca occur.

A detailed examination of the means, other than voluntary, by which Mollusca are transported from one place to another hardly comes within the scope of this work. Ocean currents, rivers, floods, cyclonic storms of wind, birds, and even beetles and frogs, play a part, more or less considerable, in carrying living Mollusca or their ova, either separately or in connexion with floating débris of every kind, to a distance from their native home. Accidental locomotion, of one or other of these kinds, combined with the well-known tenacity of life in many species (p. 37), may have contributed to enlarge the area of distribution in many cases, especially in the tropics, where the forces of nature are more vigorous than in our latitudes. The ease with which species are accidentally spread by man increases the probability of such cases occurring without the intervention of human agency, and numbers of instances may be collected of their actual occurrence.[358]

A point, however, which more concerns us here is to remark on the exceedingly wide distribution of the prevailing forms of fresh-water Mollusca. It might have been expected that the area of distribution in the fresh-water forms would be greatly restricted, since they cannot migrate across the land from one piece of water to another, and since the barriers between pond and pond, lake and lake, and one river system and another are, as far as they are concerned, all but insuperable. We might have expected, therefore, as Darwin and Wallace have remarked, to find a great multiplicity of species confined to very restricted areas, since the possibility of communication with the parent stock appears, in any given case, to be so exceedingly remote.

As is well known, the exact reverse occurs. The range, not merely of genera, but even of individual species, is astonishingly wide. This is especially the case with regard to the Pulmonata and Pelecypoda. The genera _Limnaea_, _Planorbis_, _Physa_, _Ancylus_, _Unio_, and _Cyclas_ are world-wide. Out of about ten genera of fresh-water Mollusca in New Zealand, one of the most isolated districts known, only one is peculiar. In South Africa and the Antilles no genus is peculiar. In the latter case, this fact is remarkable, when we consider that the same sub-region has at least ten peculiar genera of operculate land Mollusca alone.

To give a few instances of the distribution of particular species:--

_Limnaea stagnalis_ L. occurs in the whole of Europe, and northern Asia to Amoorland, Turkestan, Afghanistan, North Persia, and Kashmir; Greenland, North America from the Atlantic to the Pacific, and from North Canada and British Columbia as far south as Texas. The distribution of _L. peregra_ Müll., _L. truncatula_ Müll., and _L. palustris_ Müll, is almost equally wide.

_Planorbis albus_ occurs in the whole of Europe, and northern Asia to Amoorland, Kamschatka, and Japan; Turkestan, the Altai-Baikal district, Alaska and Greenland, North Canada, and the whole of eastern North America.

The distribution of _Anodonta anatina_ L., _Cyclas cornea_ L., and _Pisidium pusillum_ Gmel. is almost equally wide.

It is evident that the accidental means of transport mentioned above are insufficient to account for the facts as we find them; we are therefore compelled to seek for further explanation. Anything in the nature of a current furnishes a ready means of transport for Mollusca which have obtained a footing in the upper waters of a river, and there is no difficulty in imagining the gradual spread of species, through the agency of floods or otherwise, over a whole river system, when once established at any point upon it. The feeble clinging power of newly-hatched _Limnaea_ has often been noticed as contributing to the chances of their range of distribution becoming extended. Fresh-water Mollusca, too, or their ova, are exceedingly likely, from their extreme abundance, to be transported by water-birds, which fly without alighting from one piece of water to another. Again, the isolation of one river system from another is, in many instances, by no means well marked or permanent, and a very slight alteration of level will frequently have the effect of diverting the supplies of one watershed into another. When we know what enormous oscillations in level have taken place over practically the whole surface of the globe, we can recognise the probability that the whole river system of the earth has been mixed up and reconstructed again and again, with a very thorough blending of adjacent fauna.

It is possible that the very uniform conditions under which fresh-water Mollusca live may have something to do with the uniformity of their distribution and the comparative sameness in their development. There can scarcely be any question that the environments of fresh-water species are in themselves less varied and less liable to fluctuation than those of species whose home is the land. Water is very like water, all the world over; it may be running or motionless, warm or cold, clear or muddy, but the general tendency is for it to be free from extremes of any kind. Even if the surface water of a lake or river freezes, or becomes unusually hot, there is generally plenty of water at a lower stratum which maintains a less extreme temperature, and to which creatures can retire on the first symptoms of a change. From this two results will follow. Not only will the inhabitants of a piece of water not be inclined to vary much from the type, since their whole surroundings, food, etc., continue very much the same, but, if transported by any accident or cataclysm elsewhere, they will be exceedingly likely to arrive at a place which closely resembles their former home in all essentials. Thus the tendency for new types to be formed would be constantly checked, or rather would very seldom arise.

Mr. Belt, while recognising the importance of changes of level as affecting the distribution of fresh-water species, appears to regard the operations of such changes from a rather different point of view to that described above. “I think it probable,” he writes,[359] “that the variation of fresh-water species of animals and plants has been constantly checked by the want of continuity of lakes and rivers in time and space. In the great oscillation of the surface of the earth, of which geologists find so many proofs, every fresh-water area has again and again been destroyed.... Thus species of restricted range were always exposed to destruction, because their habitat was temporary and their retreat impossible, and only families of wide distribution could be preserved.”

* * * * *

The terrestrial surface of the globe has been divided, as indicating the facts of geographical distribution, into six regions--the Palaearctic, Oriental, Australasian, Ethiopian, Nearctic, and Neotropical. To these is sometimes added a seventh, the Neantarctic, consisting of Chili and Patagonia (and certain islands of the south Atlantic); but since the Mollusca of Chili unmistakably form a part of the Neotropical fauna, it seems hardly worth while to recognise a separate region for those of the extreme south of South America, which have no peculiar characteristics.

In certain points the exact limits of these regions, as indicated by the Mollusca, will probably not correspond to those which are marked out by other zoological classes. Wallace’s line, for instance, does not exist, as far as the Mollusca are concerned.

These regions may be further subdivided into sub-regions, thus:--

Regions Sub-regions

{ Septentrional
=Palaearctic= { Mediterranean
{ Central Asiatic

=Oriental= { Indo-Malay
{ Chinese

{ Papuan
=Australasian= { Australian
{ Polynesian

{ Central African
=Ethiopian= { South African
{ Malagasy

{ American
=Nearctic= { Californian

{ Antillean
{ Central American
=Neotropical= { Colombian
{ Brazilian
{ Chilian

=A. The Palaearctic Region=

The southern boundary of this region is the northern limit of the African Sahara, the Mediterranean forming no break whatever in its continuity. In Asia this boundary is less well marked, but roughly corresponds to the southernmost of the vast ranges of mountains which border the great tablelands of central Asia. Across Africa the line of desert is well defined; but in the north-east, as the desert approaches more nearly to the sea, the African extent of the region is correspondingly narrowed until it becomes little more than a strip of coast land, scarcely widening even in Lower Egypt. On the Morocco coast, Palaearctic land forms penetrate as far south as Cape Nun.[360] At its eastern extremity the line becomes less well defined, but probably proceeds along the snowy mountains west of Setchouan, the Pe-ling and Tan-sia-shan ranges, so as to include all the high ground of Thibet and of the upper waters of the Hoang-ho, and ultimately reaches its eastern limit at some point on the shores of the Sea of Japan.

The region thus includes all Europe, Africa north of the Sahara, with the Atlantic islands (the Azores, Canaries, etc.), North Arabia, Asiatic Turkey, the greater part of Persia, Afghanistan, Thibet, all Asiatic Russia, and a very large portion of the Chinese empire.

The principal characteristics of the region as a whole are:--

(1) The rich development of _Helix_, _Arion_, _Limax_, _Buliminus_, and _Clausilia_.

(2) The comparative absence of land operculates (see map, _frontispiece_).

(3) The uniform character of the fresh-water fauna.

It is in the southern portion of the region that _Helix_ (in the sub-genera _Macularia_, _Iberus_, _Pomatia_, and _Xerophila_) and _Buliminus_ (_Zebrina_, _Chondrula_, _Ena_) attain their maximum. In the north, _Fruticicola_ is the characteristic group; in the mountainous districts of the south-east, _Campylaea_, with _Clausilia_. The Arionidae have their headquarters in the damp and warm regions of western Europe, but are rare in the south. They only approach the Mediterranean coast in Algeria, near Gibraltar, and in the region between the base of the Pyrenees and the Maritime Alps, and are very poor in species throughout Italy and Sardinia. They are absent from almost the whole of northern Africa, the Mediterranean islands (except Sardinia), the whole Balkan district, the Crimea, Caucasus, and western Asia.[361]

The uniformity of the fresh-water fauna is disturbed only in the extreme south. A few species of _Melanopsis_, with _Neritina_, occur in southern Spain and Austria, Galicia, and southern Russia, while a _Melania_ or two (absent from Spain) penetrate the south-eastern parts of Europe as far as Germany. _Cyrena_ begins to replace _Cyclas_ in southern Russia and the Caucasus.

The Palaearctic region falls into three sub-regions:--

(1) The =Northern or Septentrional Sub-region=, _i.e._ the district north of the line formed by the Pyrenees,[362] Alps, Carpathians, and which, passing to the northward of the Aralo-Caspian district, follows the great central mountain range of Asia until it reaches the Sea of Japan, perhaps somewhere in the neighbourhood of Vladivostok.

(2) The =Mediterranean Sub-region=, _i.e._ the countries bordering on the Mediterranean, the Black and Caspian Seas, with the Atlantic Islands.

(3) The =Central Asiatic Sub-region=, _i.e._ Turkestan, Afghanistan, Thibet, and probably the districts of Mongolia and Manchuria.[363]

(1) The =Septentrional Sub-region= has been divided by some writers into two provinces, the European and the Siberian. There seems, on the whole, but little occasion to separate off northern Asia, the characteristic of which is, as will be seen below, rather the gradual disappearance, as we proceed eastward, of European species and genera, than the development of any new and peculiar groups. The remarkable fauna of Lake Baikal stands apart, not only from European, but also from the Siberian types occurring in its immediate neighbourhood.

On the whole, the Septentrional Sub-region is poor in species except those which inhabit fresh water. This fact is probably due to the extreme vicissitudes of temperature which prevail, and it is interesting to notice that the number of land Mollusca appears to touch its lowest point in districts where the annual range of temperature is greatest. On the other hand, in the western portions of the region, where the climate is moist and temperature more equable, the Mollusca are considerably more abundant and varied.

The line which separates the Septentrional from the Mediterranean Sub-region must of necessity be very roughly drawn, and stragglers from the south will be found to make their way northward, and _vice versâ_, under favouring circumstances of temperature and geological formation. Jordan has noticed[364] that species which in southern countries are not confined to any particular quality of soil are in more northern latitudes found only on limestone, which absorbs more heat than other formations. Conversely, the higher elevations of the Alps, Pyrenees, and even Carpathians are like islands in a sea, and support a thoroughly northern fauna, quite strange to that of the plains below. Thus _Helix harpa_ Say, a completely boreal shell, which is at home in Canada, Sweden, Lapland, and the Amoor district, is found on the Riffel Alp, at a height of 6000 feet.[365] _Vertigo arctica_ Wall., a species abundant in Lapland, North Siberia, Iceland, and Greenland, occurs on the high Alps of the Tyrol.

_Circumpolar Species._--A certain number of species are common to the extreme north both of the Palaearctic and Nearctic regions, and are, in fact, circumpolar. The number of these species, however, is so small, not exceeding about 40 species (= 16 genera), that it seems hardly worth while creating a special sub-region for their reception, particularly as no genus is peculiar. At the same time the fact is instructive as illustrating the close connexion of the northern districts of the two regions, a connexion which was no doubt more intimate in recent geological times than it is now.

The circumpolar genera are as follows. The list decisively sets forth the superior hardiness of the fresh-water as compared with the land genera:--

Valvata 1 sp.
Bithynia 1 „
Vitrina 1 „
Hyalinia 4 „
Helix 2 „
Patula 2 „
Pupa 3 „
Cionella 1 „
Succinea 1 „
Limnaea 7 „
Planorbis 5 „
Aplecta 1 „
Physa 1 „
Anodonta 1 „
Unio 1 „
Pisidium 1 „

_Great Britain._--There are in all about 130 species--83 land, 46 fresh-water; _Limnaea involuta_ (mountain tarn near Killarney) appears to be the only peculiar species. There are 11 _Hyalinia_, 5 _Arion_, and 25 _Helix_, the latter belonging principally to the sub-genera _Xerophila_, _Tachea_, _Trichia_, and _Fruticicola_. Three _Testacella_ are probably not indigenous, but are now so well established as to reckon in the total. Of the four _Clausilia_ two reach Ireland and one Scotland; two do not occur north of the Forth. There are only two land operculates, one of which (_Cyclostoma elegans_) occurs in Ireland but not in Scotland, while the other (_Acicula lineata_) reaches the southern counties of Scotland. Several species, _e.g._ _Helix pomatia_, _H. obvoluta_, _H. revelata_, _H. cartusiana_, _H. pisana_, _Buliminus montanus_, are restricted to the more southern or western counties; _Geomalacus maculosus_ is confined to a district in south-western Ireland.

The Pleistocene beds of East Anglia contain a number of species now extinct in these islands, whose occurrence appears to indicate a warmer climate than the present. Such are _Helix ruderata_, _H. fruticum_, _H. incarnata_, _Clausilia pumila_, _Unio littoralis_, _Hydrobia marginata_, and _Corbicula fluminalis_.

_Scandinavian Peninsula._--From Norway 121 species in all are recorded, and 148 from Sweden. The milder climate of Norway allows many species to reach a considerably higher latitude than in Sweden, thus in Sweden _Limax maximus_ only reaches 62°, but in Norway 66° 50´. Similarly _Arion hortensis_ and _Balea perversa_ only reach 63° and 61° respectively in Sweden, but in Norway are found as far north as 69° and 67° 50´. _Clausilia_ is represented by 9 species in southern Norway; one of these is found north of the Arctic circle. There are 4 _Pupa_, 9 _Vertigo_, and 11 _Hyalinia_, but _Helix_ dwindles to 14, 9 of which occur north of the Arctic circle. No land operculates are found; _Cyclostoma elegans_, however, occurs in Jutland and Zealand, which practically form a part of this district.

_Iceland._--Eleven species, all Scandinavian, occur. These are _Arion_ 2, _Limax_ 1, _Helix_ 2 (_arbustorum_ L. and _hortensis_ Müll., the latter being found only on the warmer southern coast), _Limnaea_ 1, _Planorbis_ 1, _Pisidium_ 4.

_France._--The northern, central, and eastern districts belong to this sub-region, while the southern and western, in which an entirely new element occurs and many northern forms disappear, belong to the Mediterranean. Thus, for instance, _Helix pomatia_ L., _H. incarnata_ Müll., _H. fruticum_ Müll., _H. cantiana_ Mont., _H. strigella_ Drap., _H. rufescens_ Penn., _H. plebeia_ Drap., are not found in southern France. No detailed enumeration of species is at present possible, the efforts of a large number of the leading French authorities being directed to indiscriminate species-making rather than to the careful comparison of allied forms. Perhaps the principal difference between the Mollusca of northern France and those of our own islands is the occurrence of two species of _Pomatias_. In the more elevated districts of eastern France (the Vosges, Jura, western Alps), a certain number of species occur which are confined to the high grounds of south central Europe. Among these are _Helix holoserica_ Stud., _H. personata_ Lam., _H. bidens_ Chem., _H. depilata_ Drap., _H. cobresiana_ Alt., _H. alpina_ Faure.

The Pleistocene deposits of the valley of the Somme tell the same tale as those of eastern England, containing as they do species and even genera whose northern range is now much more limited. The Eocene fossils from the Paris beds show most remarkable relationships to genera now existing in the West Indies and Central America. Others again indicate affinities with India. Thus we find _Ceres_, _Megalomastoma_, and _Tudora_ by the side of _Leptopoma_, _Faunus_, and _Paludomus_.

_Germany._--The Mollusca of the plains of northern Germany are few and not striking, and exhibit little difference from those of our own islands. In the mountainous districts of the south and south-east, a number of new forms occur, amongst which are 3 species of _Daudebardia_, a remarkable carnivorous form, with the general appearance of a _Vitrina_; 24 of _Clausilia_, many _Pupa_, several _Buliminus_, 3 of the _Campylaea_ group of _Helix_, stragglers from the Italo-Dalmatian fauna, and 1 of _Zonites_ proper. Our familiar _Helix aspersa_ is entirely absent from Germany. There are only 4 land operculates--_Pomatias_ 2, _Acicula_ 1, _Cyclostoma_ 1, all of which occur exclusively in the south. _Bithynella_ and _Vitrella_, two minute forms of fresh-water operculates akin to _Hydrobia_, occur throughout the district.

_Northern Russia and Siberia._--This vast tract extends from eastern Germany to the Amoor district. It is exceedingly poor in Mollusca, and is chiefly characterised by the gradual disappearance, as we proceed eastward, of European species. There are a few characteristic Siberian Mollusca, closely allied to European forms, and in the extreme east a new element is introduced in the appearance of types which indicate Chinese affinities. The whole district may be regarded as bounded to the south by a line drawn from Lemberg to Moscow, and thence to Perm; passing south of the Ural mountains, it includes the whole basins of the rivers Obi, Yenesei, and Lena, coinciding with the vast mountain ranges which terminate to the north the table-land of central Asia, at the eastern extremity of which it dips sharply southwards, so as to include the Amoor basin and Corea.

All the larger Helices are wanting, and no land operculates occur. _Helix arbustorum_ L., _H. nemoralis_ Müll., _H. lapicida_ L., _H. aculeata_ Müll., and _Hyalinia nitidula_ Drap., do not appear to occur east of the Baltic; _Arion fuscus_ Müll., _Helix strigella_ Drap., _Buliminus obscurus_ Müll., _Clausilia laminata_ Mont., _C. bidentata_ Bttg., _C. plicatula_ Drap., _Viviparus fasciatus_ Müll., and _Neritina fluviatilis_ L., do not pass the Urals.

In the Obi district (West Siberia) a further batch of European species find their easterly limit. Among these are _Helix hispida_ L., _Bithynia tentaculata_ L., _Vivipara vivipara_ L., _Pisidium amnicum_ Müll., and _Unio tumidus_ Retz. A few distinctly Siberian species now appear, _e.g._ _Ancylus sibiricus_ Gerst., _Valvata sibirica_ Midd., and _Vitrina rugulosa_ Koch.

The following are among the European species which reach eastern Siberia: _Hyalinia nitida_ Müll., _Succinea oblonga_ Drap., _Planorbis vortex_ L., _spirorbis_ L., _marginatus_ Drap., _rotundatus_ Poir., _fontanus_ Light., _Valvata piscinalis_ Müll., _Bithynia ventricosa_ Leach, and _Anodonta variabilis_ Drap. Here first occur such characteristic species as _Physa sibirica_ West., _P. aenigma_ West., _Helix pauper_ Gld., _H. Stuxbergi_ West., _H. Nordenskiöldi_ West., _Planorbis borealis_ Lov., _Valvata aliena_ West., _Cyclas nitida_ Cless., and _C. levinodis_ West. In the Amoor district a decided Chinese element makes its appearance in a few hardy forms which have penetrated northward, _e.g._ _Philomycus bilineatus_ Bens., and a few each of the _Fruticicola_ (Chinese) and _Acusta_ groups of _Helix_. Out of 53 species, however, enumerated from this district, as many as 33, belonging to 18 genera, occur also in Great Britain.

_Lake Baikal._--The Mollusca of Lake Baikal exhibit distinct characteristics of their own, which seem to indicate the long-continued existence of the lake in its present condition. Several entirely peculiar genera occur, which are specialised forms of _Hydrobia_, _e.g._ _Baikalia_, _Liobaikalia_, _Gerstfeldtia_, _Dybowskia_, and _Maackia_; _Benedictia_ alone extends to the basin of the Amoor. _Choanomphalus_, another peculiar and ultra-dextral (p. 250) genus belonging to the Limnaeidae, appears to be related to the West American _Carinifex_.

(2) The =Mediterranean Sub-region= is divided into four provinces: (_a_) The Mediterranean province proper; (_b_) the Pontic; (_c_) the Caucasian; and (_d_) the Atlantidean province.

(_a_) The _Mediterranean province proper_ is best considered by further subdividing it, with Fischer and others, into separate districts, each of which has certain peculiar characteristics.

(i) The _Hispano-Algerian_ district includes the greater part of the Iberian peninsula, the Balearic Islands, and northern Africa from Morocco to Tunis. The extreme western parts of these districts, including West Morocco, Portugal, Asturias, and south-west France, under the influence of the moist climate caused by the Atlantic, show some peculiar features which, in the view of some, are sufficient to justify their separation from the rest of the Hispano-Algerian portion. Among these is a marked development of the slugs, _Testacella_, _Arion_, and _Geomalacus_, the latter of which occurs even in south-western Ireland.

_Spain._--The principal features are the development of the _Macularia_, _Iberus_, and _Gonostoma_ groups of _Helix_, and the occurrence of the remarkable slug _Parmacella_, which is found in many other parts of the sub-region, and extends eastward as far as Afghanistan. _Clausilia_ has but few species, mostly in the north. There are four species of land operculates, one of which is referred to a genus (_Tudora_) now living only in the West Indies, but which occurs in the Eocene fossils of the Paris basin. In the south there are several species of _Melanopsis_ and _Neritina_.

The _States of Northern Africa_ have a thoroughly Mediterranean fauna, whose facies on the whole shows rather more affinity to Spain than to Sicily. The Helices of Morocco and Algeria belong to the same groups as those of southern Spain. Many are of a dead white colour, the better to resist the scorching effect of the sun. _Ferussacia_ is abundant, _Geomalacus_ and _Parmacella_ are represented by a single species each, and there is one _Clausilia_. According to Kobelt,[366] the original land connexion between southern Spain and Morocco must have been much more extensive than is usually assumed, and probably reached at least to the meridian of Oran and Cartagena. The Mollusca of Oran and Cartagena are, according to him, much more closely related than those of Oran and Tangier, or those of Cartagena and Gibraltar, but at Cartagena some species, which are characteristic of the Mediterranean coasts from Syria westward, disappear, are absent from the rest of Spain and from Morocco, but reappear on the south-western coasts of France. These species may possibly have pushed along that arm of the sea which, when the Straits of Gibraltar were closed as far as the latitude of Oran and Cartagena, united in comparatively recent times the Bay of Biscay with the Gulf of Lions.

The following genera, which do not occur in Spain, have probably spread into northern Africa as far as Algeria, _via_ Sicily and Tunis, namely, _Glandina_ (1 sp.), _Daudebardia_ (1 sp.), _Pomatias_ (2 sp.). Tunis shows strong traces of Sicilian influence, and Kobelt found a colony of snails, of Sicilian affinities, as far west as Tetuan.

_The Sahara._--The Algerian Sahara contains, in many places, a sub-fossil Molluscan fauna which appears to show that the district has, in quite recent times, undergone a gradual desiccation. The species are mainly fresh-water, including _Melania_, _Melanopsis_, and _Corbicula_, with here and there valves of _Cardium edule_, and indicate, on the whole, an affinity with recent Egyptian, rather than North African species. It is probable that a vast series of _étangs_, or brackish-water lakes, once stretched along this region, and were ultimately connected with the sea somewhere between Tunis and Egypt.

(ii) _Southern France._--The southern portion of France bordering on the Mediterranean contains many species, especially of _Helix_, which do not occur in the centre and north. Amongst these are--

Leucochroa candidissima Drap.
Hyalinia olivetorum Gmel.
Zonites algirus L.
Helix rangiana Desh.
„ serpentina Fér.
„ niciensis Fér.
„ splendida Drap.
„ vermiculata Müll.
„ melanostoma Drap.
„ aperta Born.
„ ciliata Ven.
„ explanata Müll.
„ apicina Lam.
„ cespitum Drap.
„ Terverii Mich.
„ pyramidata Drap.
„ trochoides Poir.
Ferussacia folliculus Gron.
Rumina decollata L.
Pupa megacheilos C. and J.

Several species of fresh-water _Hydrobia_ (_Bithynella_) occur. The district, on the whole, unites certain characteristics derived from northern Italy with those of eastern Spain.

(iii) The _Italo-Dalmatian_ district includes Italy and the neighbouring islands (Corsica, Sardinia, Sicily, Malta), and the regions at the head and north-eastern shores of the Adriatic (Carinthia, Carniola, Croatia, and Dalmatia), the line which separates these latter districts from the fauna of southern Austria, Bosnia, and Servia being very difficult to define.

_Italy_, with the neighbouring islands, has a rich molluscan fauna. In the sub-Alpine districts of northern Italy the prominent _Helix_ groups are _Campylaea_, _Pomatia_, and _Anchistoma_, which in the south are generally replaced by _Iberus_, which here attains its maximum development. Large _Hyalinia_ are abundant in the north, and _Pomatias_ and _Clausilia_ are frequent all along the Apennines. _Sicily_ has about 250 species, half of which are peculiar. Helices of the _Iberus_ type abound, but _Campylaea_ is reduced to two species. Many peculiar forms of _Clausilia_ occur, especially a latticed type of great beauty. _Ferussacia_ and _Pupa_ are well represented, and there are one each of _Glandina_ and _Daudebardia_.

_Dalmatia_ and the adjacent districts are chiefly remarkable for the rich development of _Clausilia_, which here attains its maximum (nearly 100 species). The _Campylaea_ section of _Helix_ is represented by its handsomest forms, many of which are studded with short hairs. Here too is the headquarters of _Zonites_ proper, which stretches westward as far as Provence, and eastward to Asia Minor; and also of the single European _Glandina_, which has a similar eastward range, but spreads westward through Italy and Sicily to Algeria, not occurring in southern France. The land operculates are chiefly represented by _Pomatias_, and among the fresh-water operculates are a _Melania_ and a _Lithoglyphus_, the latter having probably spread from the basin of the Danube.

(iv) The _Egypto-Syrian_ district extends along the south-eastern shores of the Mediterranean from Tripoli to North Syria, and eastward to the Euphrates valley. Lower Egypt alone belongs to this portion, the fauna of Upper Egypt being of an entirely tropical character, and belonging to the Ethiopian Region.

_Lower Egypt._--The Mollusca of Lower Egypt stand in the unique position of belonging, half to the Palaearctic, and half to the Ethiopian Region. The land Mollusca are of a distinctly Mediterranean type, while the fresh-water, directly connected as they are by the great highway of the Nile with regions much farther south, contain a large admixture of thoroughly tropical genera (_Ampullaria_, _Lanistes_, _Melania_, _Cleopatra_, _Corbicula_, _Cyrena_, _Iridina_, _Spatha_, _Mutela_). The _Helices_, which are not numerous, are rather a mixture of circum-Mediterranean species than of a specially distinctive character. _H. desertorum_, however, belonging to the group _Eremophila_, is characteristic. There is a single _Parmacella_, but the physical features of the country are unfavourable to the occurrence of such genera as _Clausilia_, _Pupa_, _Hyalinia_, and the land operculates.

_Syria._--The Mollusca, especially in the more mountainous regions of the north, are much more varied and numerous than those of Egypt. _Clausilia_ is again fairly plentiful, and the Helicidae are represented by some striking forms of the sections _Levantina_, _Pomatia_, and _Nummulina_. _Leucochroa_ has several curious types with a constricted aperture, and the Agnatha are represented by _Libania_, a peculiar form of _Daudebardia_. A prominent feature is the occurrence of a number of large white _Buliminus_ of the _Petraeus_ section (Fig. 200). Land operculates appear to be absent, but _Melanopsis_ and _Neritina_ are abundant. The Dead Sea contains no Mollusca, but Lake Tiberias has a rich fauna, including the above-mentioned genera, with a _Corbicula_ and several _Unio_.

_Upper Mesopotamia_ appears to possess a mixture of Syrian and Caucasian forms, including a _Parmacella_. Lower Mesopotamia has an exceedingly poor land fauna, but is comparatively rich in fresh-water species, the growing eastern character of which is shown by the occurrence of several _Corbicula_ and _Pseudodon_, and of a _Neritina_ of a distinctly Indian type.

(_b_) The _Pontic province_ extends from Western Austria to the Sea of Azof, and includes Austria, Hungary, Roumania, the Balkan peninsula (so far as it does not form part of the Mediterranean sub-region), the islands of the Greek Archipelago, southern Russia and the Crimea, and Asia Minor. It thus practically corresponds to the whole Danube basin, together with the lands bordering on the Black Sea, except at the extreme east, which belongs to the Caucasian sub-region. Fischer separates off Greece, Asia Minor (except the northern coast-line), and the intervening islands, with Crete and Cyprus, as constituting a portion (Hellado-Anatolic) of the Mediterranean sub-region proper. These districts, however, appear to possess scarcely sufficient individuality to warrant their separate consideration.

A prominent characteristic of the Pontic Mollusca is the great abundance of _Clausilia_ and _Buliminus_. In the islands east and west of Greece _Clausilia_ forms a large proportion of the fauna, each island, however small, possessing its own peculiar forms. The Helices belong principally to the groups _Campylaea_ (which is very abundant in Austro-Hungary), _Pomatia_ (Greece and Asia Minor), and _Anchistoma_. _Macularia_ is comparatively scarce, but is represented in Greece by one very large form (_Codringtonii_ Gray). _Zonites_ proper has its metropolis in this sub-region, and the Danube basin contains one or two species of _Melania_ and _Lithoglyphus_. _Buliminus_ is abundant throughout the sub-region, in the sub-genera _Zebrina_, _Napaeus_, _Mastus_, and _Chondrula_. Several striking forms of _Zebrina_ (_Ena_) are peculiar to the Crimea.

(_c_) _The Caucasian Province._--The limits of this province can hardly be exactly defined at present. It appears, however, to include the whole line of the Caucasus range, Armenia, and North Persia.

The land Mollusca are abundant and interesting. Among the carnivorous genera are four species of _Daudebardia_, a _Glandina_, and three peculiar forms of naked slug, _Pseudomilax_, _Trigonochlamys_, and _Selenochlamys_. There is a single _Parmacella_, the same species as the Mesopotamian, and a good many forms of _Limax_. _Vitrina_ and _Hyalinia_ are well represented, and the predominant groups of _Helix_ are _Euloto_, _Cartusiana_, _Xerophila_, and _Fruticocampylaea_, the last being peculiar. _Clausilia_ and _Pupa_ are rich in species, together with _Buliminus_ of the _Chondrula_ type. One _Clausilia_ of the _Phaedusa_ section, together with a _Macrochlamys_ (Transcaspian only), a _Corbicula_, and a _Cyclotus_, show marked traces of Asiatic affinity. There is one species each of _Acicula_ and _Cyclostoma_, and one of _Pomatias_.

The Caspian Sea, like Lakes Baikal and Tanganyika, is distinguished by the possession of several remarkable and peculiar genera. The sea itself, the waters of which are brackish, is 80 feet below the level of the Black Sea, and is no doubt a relict of what formed, in earlier times, a very much larger expanse of water. Marine deposits containing fauna now characteristic of the Caspian, have been found as far north as the Samara bend of the Volga. It is probable, therefore, that in Post-pliocene times an arm of the Aralo-Caspian Sea penetrated northward up the present basin of the Volga to at least 54° N. The Kazan depression of the Volga (55° N.) also contains characteristic Caspian fossils.[367] According to Brusina,[368] the Caspian fauna, as a whole, is closely related to the Tertiary fauna of southern Europe.

Twenty-six species of univalve Mollusca, the majority being modified forms of _Hydrobia_, have been described from the Caspian, namely, _Micromelania_ (6), _Caspia_ (7), _Clessinia_ (3), _Nematurella_ (3), _Lithoglyphus_ (1), _Planorbis_ (1), _Zagrabica_ (1), _Hydrobia_ (2), _Neritina_ (2). The bivalves are mostly modified forms of _Cardium_ (_Didacna_, _Adacna_, _Monodacna_), which also occur in estuaries along the north of the Black Sea. A form of _Cardium edule_ itself occurs, and numberless varieties of the same species are found in a semi-fossil condition in the dry or half dry lake-beds, which are so abundant throughout the Aral district.

(_d_) _The Atlantidean province_ consists of the four groups of islands, the Madeiran group, the Canaries, the Azores, and the Cape Verdes.

_The Madeiran group_ contains between 140 and 150 species of Mollusca which may be regarded as indigenous, the great majority of which are peculiar. Only 11 species are common to Madeira and to the Azores, and about the same number, in spite of their much greater proximity, to Madeira and the Canaries. No less than 74 species, or almost exactly one-half, belong to _Helix_, and 9 to _Patula_. A considerable number of the Helices are not only specifically but generically peculiar, the genera bearing close relationship to those occurring in the Mediterranean region. As a rule they are small in size, but often of singular beauty of ornamentation. Various forms of _Pupa_ are exceedingly abundant (28 sp.), as is also _Ferussacia_ (12 sp.). There are also 3 _Clausilia_ (which genus occurs on this group alone), and 3 _Vitrina_ (a genus which occurs on all the groups). The land operculates are represented solely by 4 _Craspedopoma_, which is common to all the groups except the Cape Verdes.

_The Canaries_ have about 160 species, only about a dozen of which are not peculiar. As many as 75 of these belong to _Helix_ (the sub-genera being very much the same as in the Madeiran group), and 11 to _Patula_. There is 1 species of _Parmacella_ (which occurs in this group alone), and 6 of _Vitrina_, of considerable size. A remarkable slug (_Plectrophorus_) was described from Teneriffe by Férussac many years ago, but it has never been rediscovered, and is probably mythical, or wrongly assigned. _Buliminus_ (_Napaeus_) has as many as 28 species, all but one being peculiar, and _Ferussacia_ 7. _Cyclostoma_ has two indigenous species, which, with one _Hydrocena_ and one _Craspedopoma_, make up the operculate land fauna.

_The Azores_ are comparatively poor in Mollusca, having only 52 species, nearly two-thirds of which are peculiar. _Helix_ has 15 species, _Patula_ 4, and _Pupa_ 8. _Ferussacia_, so abundant in Madeira and the Canaries, is entirely absent, its place being taken by _Napaeus_ (7 sp.), which is curiously absent from Madeira, but richly represented in the Canaries. There are 7 _Vitrina_, while the land operculates consist of one each of _Craspedopoma_ and _Hydrocena_. A singular slug (_Plutonia_), with an ancyliform internal shell, is said to occur. The group was long believed to possess no fresh-water Mollusca, but two species (one each of _Pisidium_ and _Physa_) have recently been discovered.

_The Cape Verdes_, owing to the extreme dryness of their climate, are poor in land Mollusca. There are 11 _Helix_, nearly all of which belong to the group _Leptaxis_, which is common to Madeira and the Canaries. _Ferussacia_ is absent, _Buliminus_ is represented by a single species, and there are no land operculates. Ethiopian influence, however, as might be expected from the situation of the group, is seen in the occurrence of an _Ennea_, a _Melania_, and an _Isidora_.

It will be noticed how little countenance the molluscan fauna of these island groups gives to any theory of an Atlantis, any theory which regards the islands as the remains of a western continent now sunk beneath the ocean. Had ‘Atlantis’ ever existed, we should have expected to find a considerable proportion of the Mollusca common to all the groups, and perhaps to Europe as well, and there would apparently be no reason why a genus which occurred in one group should not occur in all. As a fact, we find the species extremely localised throughout, and genera occur and fail to occur in a particular group without any obvious reason. All the evidence tends to show that the islands are purely oceanic, and have been colonised from the western coasts of the Mediterranean, _i.e._ from the direction of the prevailing currents and winds.

(3) =Central-Asiatic Sub-region.=--The countries included in this vast sub-region are Turkestan, Songaria, Afghanistan, including the Pamirs, Western Thibet, and probably Mongolia. Kashmir belongs to the Indian fauna. At present the whole district is very imperfectly known; indeed, it is only at a few points that anything like a thorough investigation of the fauna has been made. It is therefore almost premature to pronounce any decided opinion upon the Mollusca, but all the evidence at present to hand tends to show that they belong to the Palaearctic and not to the Oriental system. This is especially the case with regard to the fresh-water Mollusca, many of which are specifically identical with those occurring in our own islands. A slight admixture of such widely distributed types as _Corbicula_ and _Melania_ occurs, but it is not sufficient to disturb the general European facies of the whole. It is possible that eventually the whole district may be regarded as a sub-region combining certain characteristics of the eastern portions of the Mediterranean basin with an extension of the septentrional element, due to higher elevation and more rigorous climate. The principal features in the land Mollusca appear to be the occurrence of a number of _Buliminus_ of the _Napaeus_ group, a few _Parmacella_ (Afghanistan being the limit of the genus eastward), _Clausilia_, _Pupa_, _Limax_, and _Helix_, with several stray species of _Macrochlamys_.

=B. The Oriental or Palaeotropical Region=

This region includes all Asia to the south of the boundary of the Palaearctic region, that is to say, India, with Ceylon, Burmah, Siam, and the whole of the Malay Peninsula, China proper, with Hainan and Formosa, and Japan south of Yesso. It also includes the Andamans and Nicobars, and the whole of Malaysia, with the Philippines, as far eastward as, and including Celebes with the Xulla Is., and the string of islands south of the Banda Sea up to the Ké Is. The Moluccas, in their two groups, are intermediate between the Oriental and Australasian regions.

In this vast extent of land two distinct centres of influence are prominent--the Indian and the Chinese. Each is of marked individuality, but they differ in this essential point, that while the Chinese element is decidedly restricted in area, being, in fact, more or less confined to China itself and the adjacent islands, the Indian element, on the other hand, extends far beyond continental Asia, and embraces all the Malay islands to their farthest eastward extent, until it becomes overpowered by the Papuan and Australian fauna. Upper Burmah, with Siam, forms a sort of meeting-point of the two elements, which here intermingle in such a way that no very definite line of demarcation can be drawn between them.

Thus we have--

{ { (_a_) Indian Province
{ 1. _Indo-Malay Sub-Region_ { (_b_) Siamese Province
=Oriental Region= { { (_c_) Malay Province
{ { (_d_) Philippine Province
{
{ { (_a_) Chinese Province
{ 2. _Chinese Sub-Region_ { (_b_) Japanese Province

The Indo-Malay fauna spreads eastward from its metropolis, but has practically no westward extension, or only such as may be traced on the eastern coasts of Africa and the off-lying islands. There appears to exist no other case in the world where the metropolis of a fauna is so plainly indicated, or where it lies, not near the centre, but at one of the ends of the whole area of distribution.

Comparing the two sub-regions, the Chinese is distinguished by the great predominance of _Helix_, while in the Indo-Malay sub-region _Nanina_ and the related genera are in the ascendancy. In India itself there are only 6 genera of true Helicidae, poorly represented in point of numbers; in China there are at least three times this amount, most of them abundant in species. The Indo-Malay sub-region, on the other hand, is the metropolis of the Naninidae, which abound both in genera and species. In the Chinese sub-region _Clausilia_ attains a development almost rivalling that of S.E. Europe, while in India there are scarcely a dozen species. A marked feature of the Indo-Malay sub-region is the singular group of tubed land operculates (_Opisthoporus_, _Pterocyclus_, etc.). In China the group is only represented by stragglers of Indian derivation, while the land operculate fauna, as a whole, is distinctly inferior to the Indian. Another characteristic group of the Indo-Malay region is _Amphidromus_, with its gaudily painted and often sinistral shell; the genus is entirely absent from China proper and Japan, where its place is taken by various small forms of the _Buliminus_ group. Fresh-water Mollusca, especially the bivalves and operculates, are far more abundant in the Chinese sub-region than in the Indo-Malay.

(1) The =Indo-Malay Sub-region=.--(_a_) _The Indian Province_ proper includes the peninsula of Hindostan, together with Assam and Upper and Lower Burmah. To the east and extreme north-east, the boundaries of the province are ill-defined, and the fauna gradually assimilates with the Siamese on the one hand and the Chinese on the other. Roughly speaking, the line of demarcation follows the mountain ranges which separate Burmese from Chinese territory, but the debatable ground is of wide extent, and Yunnan, the first Chinese province over the border, has many species common with Upper Burmah.

The gigantic ranges of mountains which bound the sub-region to the north-west and north limit the extension of the Indian fauna in those directions in a most decisive manner. There is no quarter of the world, even in W. America, where a mountain chain has equal effect in barring back a fauna. In the north of Kashmir, where the great forests end, there is a most complete change of environment as the traveller gains the summit of the watershed; but Kashmir itself distinctly belongs to the Indian and not the Palaearctic system. The great desert to the south of the Punjab is equally effective as a barrier towards the west.

The Mollusca of India proper include a very large number of interesting and remarkable genera. India is the metropolis of the great family of the Naninidae, or snails with a caudal mucus-pore, which are here represented by no less than 14 genera and over 200 species. The genera _Macrochlamys_, _Sitala_, _Kaliella_, _Ariophanta_, _Girasia_, _Austenia_, and _Durgella_ are at their maximum. _Helix_ is scarcely represented, containing only about 30 inconspicuous species (leaving Ceylon out of account). _Buliminus_ is abundant, especially in the north. The Stenogyridae are represented by _Glessula_, which is exceedingly abundant in India, but has only a few straggling representatives in the rest of the Oriental region. Among the Pupidae is the remarkable form _Boysia_, with its twisted upturned mouth, while _Lithotis_ is a peculiar form allied to _Succinea_, to which group also probably belongs _Camptonyx_, a limpet-like form with a very small spire, peculiar to the Kattiawar peninsula. _Camptoceras_, an extraordinarily elongated sinistral shell, with a loosely coiled spire, is peculiar to the N.W. Provinces.

Among the fresh-water pulmonates is an _Ampullarina_, a genus only known elsewhere from the Fiji Is. and E. Australia. _Cremnoconchus_ is a form of _Littorina_, peculiar to the W. Ghâts, which has habituated itself to a terrestrial life on moist rocks many miles from the sea. The fresh-water operculates include the peculiar forms _Mainwaringia_, from the mouth of the Ganges (intermediate between _Melania_ and _Paludomus_), _Stomatodon_, _Larina_, _Fossarulus_, _Tricula_, and others. The bivalves are neither numerous nor remarkable; _Velorita_, a genus of the Cyrenidae, is peculiar.

The land operculate fauna of India is singularly rich and varied. About 25 genera, and at least 190 species, occur. Here we find the metropolis of _Cyclophorus_ among the larger forms, and of _Diplommatina_ and _Alycaeus_ among the smaller. A large proportion of the operculate genera are quite peculiar to the extreme south of India and Ceylon. The appearance of a few species of the European genus _Pomatias_ is very remarkable.

The carnivorous genera are poorly represented. A few _Ennea_ occur, while _Streptaxis_ is practically confined to the extreme south and north-east.

_Land and Fresh-water Mollusca of India proper_