(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.