Fig. 204.—Helix (Acavus) Waltoni Reeve, Ceylon, showing embryonic shell (emb). × ⅔.

Genera and Subgenera occurring in the Cingalese District, but not in N. and Central India

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.

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.

The Andaman and Nicobar Islands possess no peculiar features in their land Mollusca. They are closely related to the adjacent coasts of Lower Burmah. Amphidromus (2 sp.) occurs in the Andamans alone, and Clausilia (2 sp.) in the Nicobars alone, while Hyalimax occurs in both groups. A remarkable Helix (codonodes Fér.) from the Nicobars appears to find its nearest relations in the isolated group from Busuanga and Mindoro (p. [315]). Land operculates are abundant, in the Nicobars actually outnumbering the pulmonates (28 to 22). Helicina and Omphalotropis, genera characteristic of small islands, are found on both groups.

(b) The Siamese Province includes the area occupied by the districts known as Siam, Laos, Cambodia, Cochin China, Annam, and Tonquin. Along the whole of its northern frontier, the zoological is no more than a political boundary, while on the east the mountain ranges which part Siam from Pegu and Tenasserim are not of sufficient height to offer any effective barrier to distribution. The province is accordingly qualified to a considerable extent by Indian and Chinese elements.

Streptaxis is, but for three Ennea, the sole representative of the carnivorous genera, and attains its maximum in the Old World. Partly owing to Chinese influence, the Helicidae, with 11 genera and 46 species, begin to regain their position as compared with the Naninidae (12 genera, 54 species). Of the Helicidae, Acusta and Hadra appear now for the first time, and, with Plectotropis, Stegodera, and Clausilia, form a marked Chinese element. Amphidromus, with 33 species, is the most characteristic land pulmonate. Several genera, whose nucleus of distribution lies among the islands farther east, appear to have penetrated as far as these coasts. Such are Chloritis, Camaena, and Obbina among the Helicidae, Trochomorpha, and, of the operculates, Helicina.