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.