The gigantic ranges of mountains which bound the sub-region to the north-west and north limit the extension of the Indian fauna in those directions in a most decisive manner. There is no quarter of the world, even in W. America, where a mountain chain has equal effect in barring back a fauna. In the north of Kashmir, where the great forests end, there is a most complete change of environment as the traveller gains the summit of the watershed; but Kashmir itself distinctly belongs to the Indian and not the Palaearctic system. The great desert to the south of the Punjab is equally effective as a barrier towards the west. The Mollusca of India proper include a very large number of interesting and remarkable genera. India is the metropolis of the great family of the Naninidae, or snails with a caudal mucus-pore, which are here represented by no less than 14 genera and over 200 species. The genera _Macrochlamys_, _Sitala_, _Kaliella_, _Ariophanta_, _Girasia_, _Austenia_, and _Durgella_ are at their maximum. _Helix_ is scarcely represented, containing only about 30 inconspicuous species (leaving Ceylon out of account). _Buliminus_ is abundant, especially in the north. The Stenogyridae are represented by _Glessula_, which is exceedingly abundant in India, but has only a few straggling representatives in the rest of the Oriental region. Among the Pupidae is the remarkable form _Boysia_, with its twisted upturned mouth, while _Lithotis_ is a peculiar form allied to _Succinea_, to which group also probably belongs _Camptonyx_, a limpet-like form with a very small spire, peculiar to the Kattiawar peninsula. _Camptoceras_, an extraordinarily elongated sinistral shell, with a loosely coiled spire, is peculiar to the N.W. Provinces. Among the fresh-water pulmonates is an _Ampullarina_, a genus only known elsewhere from the Fiji Is. and E. Australia. _Cremnoconchus_ is a form of _Littorina_, peculiar to the W. Ghâts, which has habituated itself to a terrestrial life on moist rocks many miles from the sea. The fresh-water operculates include the peculiar forms _Mainwaringia_, from the mouth of the Ganges (intermediate between _Melania_ and _Paludomus_), _Stomatodon_, _Larina_, _Fossarulus_, _Tricula_, and others. The bivalves are neither numerous nor remarkable; _Velorita_, a genus of the Cyrenidae, is peculiar. [Illustration: FIG. 202.--Characteristic Indian Mollusca: =A=, _Hypselostoma tubiferum_ Blanf.; =B=, _Camptoceras terebra_ Bens.; =C=, _Camptonyx Theobaldi_ Bens.] [Illustration: FIG. 203.--_Streptaxis Perroteti_ Pfr., Nilghiri Hills: =A=, adult; =A´=, young form.]