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:—

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:—

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—

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.