140 Records <if tlie [tididii Museum. [Vol. XIV ^ 



Pelecypoda. 



Physiinio micro pleroides. (Jorhicula noellinqi. 



Physimio ferrugineus. Fisidium c.asertamini . 



In this list of 30 species and races we have the names of 13 genera. 

 Of these, 10 are of wide distribution, one distinctly Indo-Chinese, and 

 2 peculiar to Burma. 



The widely distributed genera have, as genera, little geographical 

 interest. The characteristic Indo-Chinese genus (Physunio) is, however, 

 interesting because it represents a real element in the fauna of the Shan 

 Plateau. This genus, of which one species (P. velaris, Sow.) has pene- 

 trated as far west as Assam, and another to Sumatra, is found mainly in 

 Cambodia, Cochin China and Siam. The Inle species belong to a 

 Siamese-Cambodian section of the genus, while the Assamese form 

 represents a section of wider range. 



The endemic Burmese genus Hydrohioides bears in some respects 

 the same relation to the widely distributed Bitliynia as Taia does to the 

 still more widely distributed Vivipara. Both genera have probably 

 originated on the Shan Plateau, but have spread sparsely beyond its 

 borders. 



Of the 30 names on the list 28 are those of the formae typicae of their 

 species, while 2 are those of races or subspecies. Of the 28, 16 (more 

 than half) are those of species endemic or practically endemic on the 

 Shan Plateau, while 7 (one quarter) are those of species only known 

 from the Inle basin. It is clear, therefore, that a well-marked endemic 

 Shan element is present among the aquatic Mollusca of the district. 

 It is represented by 17 out of 30 species and races, i.e., by over 50 per 

 cent, of the whole. 



No other geographical element is so conspicuous. One Western 

 Chinese species {Limnaea andersoniana) has been found, while only two 

 (Paludomus ornatus and Ampullaria winhleyi) apart from endemic Shan 

 forms are exclusively Burmese or Burmese and Assamese, and one 

 (Vivipara lecytMs) Assamese, Burmese and Western Chinese. These four 

 species (a little less than one-seventh of the species represented in 

 the fauna) may be considered to compose together a Far Eastern 

 element. 



Two species {Melania tuherculata and Planorbis exustus) are widely 

 distributed in the Oriental region and M. tuherculata ranges far beyond 

 the limits even of that region. It is probable that three other species 

 {Planorbis calathus, P. caenosus and P. trochoideus) have also a wide 

 Oriental distribution, but their minute size has caused them to escape 

 the notice of collectors, and the records of their occurrence are few and 

 scattered. 



Evidence of the existence of a Palaearctic element, though not very 

 definite, is not altogether lacking. Pisidium casertanum has hitherto 

 been found only in the Palaearctic Region, and the form that lives in the 

 Inle Lake closely resembles those only known hitherto from the 

 eastern part of the Region, from Lake Baikal and Japan. The existence 

 of a species allied to the Tibetan Limnaea boivelli on the Shan Plateau 

 points in the same direction. 



