6o6 Records of the Indian Museum. [Vol. XXII, 



the genus Lamellidcns we have probably two groups of species, 

 in one of which the marsupiura is foimed by all the four gills and 

 in the other by the outer pair only. This may possibly be 

 correlated with the conditions under which the two gioups of 

 species are found. L. iiiarginalis is a stream-form while L. corri- 

 anus is commonly found in ponds or very sluggish streams In 

 the case of typical stream-forms it may be necessary to produce as 

 large a number of glochidia as possible as the chances of their 

 being washed away are very great, and probably in response to 

 this necessity all the four gills have taken on the marsupial func- 

 tion in these forms. 



The formation of the marsupium by all the four gills in 

 L. marginalis does not in any way affect the position assigned 

 to the genus by Ortmann and myself in the subfamily Unioninae 

 of Ortmann's classification, as the marsupium in this subfamily 

 is stated to be formed either by all the four gills or by the outer 

 pair of gills only. 



Lamellidens marginalis (Lamarck). 



1914. Laniellideiis marginalis, Simpson, op. cit., pp. 1166—1168. 

 1919. Lamellidens niarginatts. Prasliad, op. cit., p. 293, fig. 4. 



In the paper cited above my description and ficure of the 

 animal of L. marginalis was based on specimens which I, with 

 Preston, considered doubtfully to represent a variety of this spe- 

 cies. As a result of a careful study of the whole collection in the 

 MuFCum I find that these specimens really belong to /. . corrianus , 

 which I consider to be a distinct species. 



'J he description of the shell of this species in Simpson's mo- 

 nograph is fairly complete, but the following distinctive char- 

 acters may be noted. The shell is sub-elliptical with slightly in- 

 flated but not greatly elevated beaks. The dorsal slope is in most 

 specimens a little curved and the posterior wing is very narrow. 

 The hinge (fig. 29A) is formed by two lamellar pseudo-cardinals in 

 the right valve ; these are situated one below the other and the 

 lower is better developed, both, however, are in continuation of 

 the laterals ; in the left valve there is only a single pseudo-car- 

 dinal like a feebly developed ridge, simple in most specimens but 

 in a few becoming cut up by a notch into two. In the latter case, 

 owing to the inclined nature of the notch and the unequal deve- 

 lopment of the two component paits of the ridge of the anterior 

 edge, the posterior of the two teeth comes to lie at a slightly 

 lower level than the anterior tooth and this results in the produc- 

 tion of two distinct pstudo-cardinals in the left valve also. The 

 gradual evolution of the two teeth can be traced in a large series. 

 The lateral teeth are somewhat curved, there being two in the 

 left and a single one in the right valve. A trace of a second 

 lateral in the form of a minute ridge at the base of the lamellar 

 lateral of the right valve can also be seen in some fully grown 

 specimens. 



