1921.] Manipur Molluscs. 621 



possibly Sylhet) in the west across Upper Burma and China to the 

 Philippines, Formosa and Japan. The precise geographical range, 

 which at present appears discontinuous, is probably unknown. 

 Species have been found in Kashmir, the valleys of the Ganges 

 and Brahmaputra, Manipur and Japan. Our present knowledge 

 of the anatomy of the Oriental Unionidae is too incomplete to 

 render it possible to lay down exact geographical boundaries for 

 the genera, Indonnia is apparently characteristic of the eastern 

 parts of the Indian Empire, but extends well into Peninsular India, 

 while Trapezoideus probably does not occur west of Eastern Assam 

 and has its headquarters in the Indo-Chinese peninsular area. 



The genera of aquatic molluscs found in Manipur do not, there- 

 fore, provide any very clear guidance as to the origin of its aqua- 

 tic fauna, except in so far as they indicate the presence of a dis- 

 tinct Far Eastern element. Lecyihoconcha is the most noteworthy 

 in this respect. It is also noteworthy, however, that the charac- 

 teristic Burmese genera Hydrohioides, Taia and Teinnotaia have 

 not been found in Mar.ipur. 



In analysing the list of species from a geographical point 

 of view it will be as well to consider the Gastropoda and the 

 Pelecypoda separately, for they follow different rules in their 

 dispersal. There are twenty-six names of Gastropod species on the 

 list. Three of these have a very wide range in the Oriental 

 Region namely Indoplanorbis cxustus, which is common all over 

 the plains of the Indian Empire east of the Indus, Siam, the 

 Sunda Isles, etc. ; Melanoides tuberculalns , distributed practi- 

 cally all over the Ethiopian and Oriental Regions (except at high 

 altitudes) and found also in adjacent parts of the Palaearctic and 

 Australasian Regions, and Gyruiilus convexiusculits, the range of 

 which extends on the mainland from Mesopotamia to Eastern 

 China and includes a considerable part of the Malay Archipelago. 



Nine species may be called " Indian," being found both east 

 and west of the Bay of Bengal but not, or only a short distance 

 beyond, the eastern boundaries of the Indian Empire and not or 

 hardly west of the Indus. They are : — 



Amnicola orcula. . Hippeutis (?) unibilicalis. 



Acrostoina variabilis. Segmcntina calathus. 



Limnaea acuminata. Ancylus verruca. 



Limnaea ovalis. Ancylus ceylanicus . 



Gyraulus cantori. 



The first of these species is essentially Gangetic and is re- 

 placed in Peninsular India and Ceylon by a closely allied species 

 or race, A. stenothyroides (Dorirn). It has not been found in 

 Burma Limnaea acmninata and L. ovalis are found all over the 

 Indo-Gangetic plain and Peninsular India. The former is known 

 from Upper Burma ; the latter has not previously been recorded 

 from any placc east of the Bay of Bengal, and is very rare in 

 Manipur. Gyraulus cantori is a scarce species, closely related 

 to the widely distributed G. convexMscidus and at present known 



