3o6 



THE ANDAMANS AND NICOBARS 



CHAP. 



— of marine forms caught in process of habituation to a fresh- 

 water or even a land existence. 



The Andaman and Nicohar 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 Olausilia (2 sp.) in the Nicobars 

 alone, while Hyalimax occurs in both groups. A remarkable 

 Helix (^codonodes Fer.) from the Nicobars appears to find its 

 nearest relations in the isolated group from Busuanga and Min- 

 doro (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. 



(5) The Siamese Provhice 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 com- 

 pared with the Naninidae (12 genera, 54 species). Of the 

 Helicidae, Acusta and Hadra apj)ear now for the first time, 

 and, with Plectotropis^ Stegodera^ and Olausilia, form a marked 



Chinese element. Amphidromus, 

 with 33 species, is the most char- 

 acteristic land pulmonate. Several 

 genera, whose nucleus of distribu- 

 tion lies among the islands farther 

 east, appear to have penetrated as 

 far as these coasts. Such are Chlo- 

 ritis, Camaena, and Ohhiyia among 

 the Helicidae, Trochomorpha, and, 

 of the operculates, Helicina. 



Land operculates are very 

 richly developed. In all, there are 17 genera and 104 species 



Fig. 206. — Cyclophorus siamensis 

 Sowb., Siam. 



