1918.] N. Annandale : Mollvscs of the Inle Lalae. 113 



and in the degree of depression of the whole shell. In both there 

 is usually a curious protruding veil-like structure running along the 

 peripheral carination or angle. This " velum " is rarely altogether 

 absent, but I believe it to be of parasitic origin. The presence of a 

 similar structure on the shells of P. saigonensis is perhaps indicated 

 in Clessin's^ figure of the upper surface of the shell of P. compressus. 

 In well-preserved specimens it has a gelatinous appearance and is full 

 of bacteria. 



The species (both varieties) is common in all parts of the Inle Lake 

 among dense masses of living weed. 



Possibly P. velifer should be regarded simply as a lacustrine phase 

 of P. saigonensis. Its shell closely resembles subfossil specimens from 

 He-Ho, except that it is considerably smaller, has the sculpture finer 

 and more regular and appears to be thinner. The subfossil shells are 

 naturally more opaque than living ones. 



Planorbis trochoideus, Benson. 



1836. Planorbis trochoideus, Benson, op. ciL, p. 742. 



1870. Planorbis trochoideus, Hanley and Theobald, Conch. Ind., pi. xxix, figs. 4-6. 



This species occurs commonly with P. velifer in both the central 

 region and the marginal zone of the Inle Lake. It is also found in the 

 fossil lake-deposit of the He-Ho plain. Shells from the lake agree well 

 with Hanley and Theobald's figures. The largest are about 3 mm. 

 in maximum diameter. They are diaphanous but tinted with yellow. 

 I can find no trace of internal partitions such as exist in P. calathus 

 and P. caenosus. 



The species has hitherto been found only in the neighbourhood of 

 Calcutta. 



Planorbis calathus, Benson. 



1850. Planorbis Calathus, Benson, Ann. Mcuj. Nat. Hist., (2) V, p. 348. 

 1876. Planorbis calathus, Hanley and Theobald, op. cit., pi. xxix, figs. 1-3. 



P. calathus is common in the Inle Lake with the two preceding 

 species. The specimens are very small, not exceeding 3 mm. in maxi- 

 mum diameter. Some are colourless, while others are distinctly 

 brownish, but all are transparent. 



The species has a wide distribution in northern India (mainly in 

 the Himalayas) and Ceylon ; it is also recorded from Siam. 



Planorbis caenosus, Benson. 



1850. Planorbis caenosus, Benson, op. cit., p. 349. 



1876. Planorbis caenosus, Hanl«y and Theobald, op. cit., pi. xxix, figs. 7-9. 



We found two very small specimens, only 2 mm. in maximum 

 diameter, in the marginal zone of the Inle Lake of! Fort Stedman. They 

 are fully adult, as one of them contains eggs in the ovary. In spite 

 of their small size the shells agree well with Hanley and Theobald's 



^ Cicssin, " Limnaeideu " in Martini and Chemnitz's Conch. Cab. {ed. Kiister), p. 191, 

 pi. xvii, fig. 10 (1886). 



