1922 ] B. Pkashad : Burmese Unionidac. 93 



Haas erected this genus for Unio laosensis Lea in 1913, but 

 Simpson considers that the species is an undoubted Margaritana 

 and that the new genus is not justified. The genus, however, 

 appears to be well characterized and I agree with Haas in separ- 

 ating M. laosensis, with its peculiar distribution in Cambodia, 

 Siam and Burma, from the other species of the genus Margari- 

 tana. Godwin-Austen has recently described another species from 

 the Shan .States under the name M. woodlhorpi ', but of this I 

 have seen no specimens. 



Margaritanopsis laosensis (Lea). 

 PI. II, figs. 1-4. 



1863. Unio laosenis, I.ea, Proc. Acad. Nat. Sci. Philadelphia VII, 



p. 190. 

 1913. Margaritanopsis laosensis, Unas, op. nil., p. 33. 



1913. Margaritanopsis laosensis, Ha;is, op. ciL, pp. 122, 123, pi. vii, 



figs. 1, 2. 



1914. Margaritanopsis laosensis, Simpson, op. cit., pp. 520, 521. 



My reasons for agreeing with Haas in keeping this species 

 in his new genus Margaritanopsis are based on an examination of 

 four specimens collected by Fea in the Karin Hills, Burma, at an 

 altitude of 1000-1200 feet and labelled Unio sella T. Canefri, a 

 manuscript name only as the species was never described as 

 such. These specimens, as was rightly considered by Haas, are 

 referrable to this species and a^e of special interest because they 

 beautifully illustrate the changes that take place in the structure 

 of the hinge during the growth of the young into the adult shell, 

 changes which appear to be characteristic of the genus. 



The young shells are somewhat rhomboid al and only show a 

 beginning of the arcuate outline of the ventral margin of the 

 adult shells. They are thin and not at all solid. The pseudo- 

 cardinals in the right valves of the young shells are lamellar, thin, 

 and lie one above the other ; in the adult shell the upper or 

 anterior becomes very thick, somewhat knob-like and lies just next 

 to the scar of the anterior adductor muscle, the lower (or now the 

 posterior) comes to be more or less in line with the anterior and 

 is separated from it by a fairly deep groove, it now takes the form 

 of an elongated ridge with its anterior edge raised into a trigonal 

 tooth-like structure. In the left valve there is a single lamellar 

 pseudocardinal in the young shells, but in the adult it becomes 

 very thick and divided into two parts — an anterior smaller and 

 somewhat trigonal and a posterior much larger and conical, for 

 interlocking with the teeth of the other valve. I have nothing 

 further to add to Lea's original description of the species and to 

 Haas' elaborate notes on it. 



The species described as Unio rectangularis by Tapparone, 

 Canefri (loc. cit., pp. 354, 355) is based on a single very young- 

 shell. It is undoubtedly to be referred to the genus Margaritanop- 



1 Godwin-Austen, Rec. Ind. Mus. XVI, pp. 202-204, pi. xv. (1919). 



