72 



MEMOIRS OF THE CARNEGIE MUSEUM. 



become effaced, leading thus to the var. catillus (PL VI, figs. 5, 6). Again the 

 posterior angle of the shell may be less developed, and the posterior end may be 

 shorter, a condition generally connected with a deficient development of the 

 whole posterior slope, thus emphasizing the swollen anterior part of the shell. 

 In such cases, the lower margin is more curved upward behind, producing a more 

 rounded, not triangular outline, leading to the var. cordatum. The obesity of 

 the shell, although always considerable, is variable, but there are no shells which 

 could be called flat; yet in specimens with the posterior angle much drawn out, 

 the posterior section of the shell may be flat (PI. VI, fig. 8). The nacre is generaUy 

 white, but individuals are occasionally found, where it is reddish. 



Although the group of P. obliquum has been misunderstood by most authors, 

 the main species has generally been properly recognized, and the references given 

 by Simpson are correct. But the figure of Call (1900, PI. 59, upp. fig.), under the 

 name of U. solidus, should be added : it is a typical P. obliquum. 



Fui. S. 



• Plcurobcma obliquum. 

 + Pleurobenia obliquum cordatum. 



■ Pleurobema obliquum calillus. 

 X Pleurobema obliquum coccineum. 



T Pleurobema obliquum pauperculum. 



A Pleurobema obliquum rubrum. 



Localities in Pennsylvania represented in the Carnegie Museum: 



Ohio River, Smiths Ferry (W. F. Graham), Shippiagport, Cooks Ferry and Industry, Beaver Co.; Corao- 



polis (S. N. Rhoads) and Neville Island, Allegheny Co. 

 Monongahela River, Westmoreland Co., and Charleroi, Wasliington Co. (G. A. Ehrmann). 

 Allegheny River, Godfrey and Kelly, Armstrong Co. 



