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 placente are distinct, and 
seem to be persistent. Glochidia small, suboval, without hooks. 
Type P. clava (Lamarck). 
It isan 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 (Elliptio 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. PP. 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 riddell, | believe the identification is right. 
14 The species of Pleurobema described by Rafinesque are unrecognizable, but 
L. Agassiz (1852) has made this the type. 
