1920.] B. Prashad : Notes on Lamellihranchs. 173 



The specimens which I assign to this form are from the 

 Upper Brahmaputra (Tezpur), Assam, and a few from Dacca, 

 Eastern Bengal in the district in which the Ganges and the Brah- 

 maputra are closely adjacent. 



Subsp. daccacnsis (Preston). 

 (PI. IX, figs. 5-8.) 



191 2. Parreyssia daccaensis, Preston, Rec. Ind. Mas. \'II, p. 300. 



1914. Parreyssia daccaensis, Simpson, op. cit. p. 11 14. 



1915. Parreyssia daccaensis, Preston, op. cit. pp. 165, 166, figs. 16 (1-3). 



Preston's species was founded on a single 3'oung shell, but 

 in the Indian Museum collection there are now a large number of 

 specimens of this species from Dacca and Mirpur, Eastern Bengal, 

 a young shell from Bhagalpur, Bengal, and many shells from 

 Sylhet, Assam. 



The species resembles the typical form but is much shorter, 

 broader, more convex, much more swollen, with the umbones more 

 distinct and convex, with the upper margin very much more arched 

 and the hinge still more strongly developed. The pseudocardi- 

 nals and the lateral teeth are both much stouter and thicker, and 

 the former are in many cases so striate and ragged as to recall 

 the condition in the genus Parreyssia. The second or posterior 

 pseudocardinal of the left valve, which projects from the margin 

 of the beak about its middle, has become larger, somewhat sub- 

 triangular in outline and has assumed a real tooth-like shape. In 

 some shells the spout or the spatulate process on the posterior 

 margin described in the subsp. obesa is more marked than in 

 others, but in this form it is situated a little below the middle 

 line. 



It may be pointed out here that the s^'stem of hinge-teeth of 

 this form is not at all like that of Parreyssia feddeni (Theobald) 

 as Preston thought. Unfortunately Theobald's original descrip- 

 tion of the hinge of P. feddeni is incomplete in that he describes the 

 pseudocardinal as " in valve dextro singulo," whereas there is a 

 second much thinner ridge above the thick and larger lower 

 pseudocardinal. Preston did no.t point out this inaccuracy in 

 describing the teeth of his species as similar to those of P. feddeni. 



It is also of interest to note in connection with the thick-shelled 

 Unionids of the group of L. jenkinsianus and its subspecies consi- 

 dered above, that their shells are of great economic importance 

 in the provinces of Bengal and Assam. They form the greater 

 part of the raw material for the pearl-button industrj^ and are 

 also burnt in large quantities for making lime. 



