264 ANNALS OF THE CARNEGIE MUSEUM. 
hooks (see Ortmann, 19110, pl. 89, fig. 4). Length and height about 
the same: 0.15 mm. 
Color of soft parts grayish or yellowish white. Among the numerous 
specimens investigated not one has been found which showed any 
traces of orange color. 
Pleurobema obliquum (Lamarck). 
A large number of specimens from the Ohio and Allegheny in 
Pennsylvania, and the Ohio in West Virginia and Ohio have been 
investigated. Gravid females have been found only a few times in 
June. 
Structure of soft parts absolutely identical with that of P. coccineum, 
but glochidia have not been observed. 
I do not think that this form is specifically distinct from P. coc- 
cineum. It is the form of the large rivers, which is represented in the 
headwaters and smaller streams by P. coccineum. In the Allegheny 
River in Armstrong Co., Pennsylvania, these two forms are connected 
by all kinds of intergrades. 
Pleurobema pyramidatum (Lea). 
Not more than a dozen specimens of typical pyramidatum have been 
seen, found always associated with the foregoing form. Females were 
among them, but none gravid. I also received one male and three 
females of this form from Arkadelphia, Arkansas, collected by H. E. 
Wheeler. 
This is merely an extreme variety of P. obliquum, connected with 
it by frequent transitional forms, and consequently the anatomy is 
absolutely identical. 
Pleurobema clava (Lamarck). 
About twenty-five specimens, among them gravid females, have 
come under observation. They are all from the Ohio drainage in 
western Pennsylvania. This species is gravid in June and July. 
The soft parts have been described by Lea (Obs., X, 1863, p. 441), 
but only those of the male. 
Anatomy like that of the other species of Plewrobema. It should be 
mentioned that the mantle-connection between the anal and supra-anal 
is rather short, and was always found present. The anal is rather 
distinctly, but finely, papillose. Posterior margins of palpi connected 
for only a short distance. 
