264 Annals of the Carnegie Museum. 



hooks (see Ortmann, 191 lb, pi. 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 \ 'irginia 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 Pleurobema. 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. 



