The district consisting of _Upper Burmah_, _Pegu_, _Tenasserim_, and _Aracan_, while essentially a part of the Indian province, contains several Siamese genera which are not found in India proper, as well as several which are at present peculiar. Amongst the former category are, of Helicidae, a single representative each of the genera _Camaena_ (Siamese and Chinese) and _Aegista_ (Chinese). Influence of the same kind is seen in the increased numbers of _Plectopylis_ (14 sp.) and _Plectotropis_ (5 sp.), of _Clausilia_ (10 sp.) and _Amphidromus_ (5 sp.), and of the large tubed operculates (11 sp. in all). _Sesara_ and _Sophina_ among the Naninidae are strange to India, while _Hyalimax_ is common only to the Andamans, Nicobars, and Mascarene Is. _Hypselostoma_ (Fig. 202, A) is a most remarkable genus of the Pupidae, reminding one of _Anostoma_ of the New World. It is peculiar to the peninsula, but for one species in the Philippines. Among the Pupinidae, we have the peculiar _Raphaulus_ and _Hybocystis_ (Fig. 205), a very remarkable form, of which another species occurs at Perak. Two _Helicina_ mark the most westward extension of the genus on the mainland. In the extreme north of Upper Burmah, Indian and Chinese forms intermingle. [Illustration: FIG. 205.--_Hybocystis gravida_ Bens. Young and adult.] The Burmese district, together with the Indian and Siamese provinces, is pre-eminently the home of a group of Mollusca, originally of marine origin, which have permanently habituated themselves to a brackish or fresh-water existence. They belong to widely different families, and even Orders. Besides _Cremnoconchus_ mentioned above, we have, among the bivalves, _Novaculina_, a _Solen_ living in fresh water in the Ganges, Irawadi, and Tenasserim estuaries; _Scaphula_, an _Arca_, one species of which occurs in the Ganges hundreds of miles above the tide-way (see Fig. 9, p. 14); and _Martesia_, a _Pholas_ from the Irawadi Delta. _Clea_ (which also occurs in Java and Sumatra) is probably an estuarine _Cominella_; a _Tectura_ has earned the name _fluminalis_ from its exclusive residence in the Irawadi R.; _Iravadia_ is probably a _Rissoina_ of similar habits, occurring from Ceylon round to Hong-Kong; _Brotia_ is a _Cerithium_ from an affluent of the River Salwin, and _Canidia_ is a _Nassa_, occurring in the embouchures of rivers from India to Borneo. Nowhere else in the world is there such a collection--not exhausted by this list--of marine forms caught in process of habituation to a fresh-water or even a land existence.