262 Annals of the Carnegie Museum. 



prominent, with moderately deep beak-cavities; hinge-teeth well 

 developed. Outer surface without sculpture. Epidermis generally 

 lighter or darker brownish, sometimes yellowish, with more or less 

 distinct rays, which may be arranged in blotches, or may be fine lines, 

 or entirely absent. Beak-sculpture obscure, consisting of a few 

 concentric ridges, not extending upon the disk. Nacre generally 

 whitish, or red. 



Soft parts practically identical with those of Plethobasus, except 

 in color, which is generally paler, sometimes yellowish, even with a 

 suggestion of pale orange (in P. clava), but in most cases without this. 

 The eggs are, where known, white, and the placental are distinct, and 

 seem to be persistent. Glochidia small, suboval, without hooks. 



Type P. clava (Lamarck). 14 



It is an unhappy coincidence, that the type species does not represent 

 the normal condition of the genus, but a rather extreme form of it. 

 In general, we may say that Pleurobema resembles Quadrula in the 

 soft parts, and Fusconaja in the shape of the shell, with the exception 

 that it has reached the advanced stage of having only the outer gills 

 marsupial. It differs from the following genera (Elliptic- and Unio- 

 merus) only by the characters of the shell, chief among which is the 

 general outline, which is of the short, rounded, quadrate, or oblique 

 type, while Elliptio and Uniomerus have the elongate and straight 

 type of shell. Simpson (1900&, p. 760, footnote 2) had difficulty in 

 defining this genus, and there are indeed species which are to a degree 

 intermediate between Pleurobema and Elliptio. P. clava is a peculiar 

 type. Since I have not had any opportunity to investigate these 

 doubtful forms, I have made the present arrangement of the genera 

 to suit the material at hand, but I shall not be astonished if further 

 studies will necessitate changes. 



Pleurobema riddelli (Lea). 



One young male, and one larger female (sterile) are before me, from 

 Pearl River, Jackson, Hinds Co., Mississippi, collected by A. A. 

 Hinkley. 



I was a little uncertain about the identification of this species, but 

 since Mr. Walker, to whom I sent these specimens, also thinks that 

 they are riddelli, I believe the identification is right. 



14 The species of Pleurobema described by Rafinesque are unrecognizable, but 

 L. Agassiz (1852) has made this the type. 



