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.