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.