354 MEMOIRS OF THE CARNEGIE MUSEUM. 



EXPLANATION OF PLATE VI. 



All figures are three-fourths natural size, a, lateral view; h, dorsal view. 



Fig. 1. Plethobasus cyphyus (Rafinesque). Same specimen as the one figured on 

 Plate V, fig. 6, from the Allegheny River, Kelly Armstrong Co., Pennsylvania, collected 

 September 27, 1907; C. M. Cat. No. 61.3065. 



Fig. 2. Plethobasus cyphyus (Rafinesque). Specimen of medium size, and rather 

 short, from the Allegheny River, Kelly, Armstrong Co., Pennsylvania, collected Sep- 

 tember 27, 1907; C. M. Cat. No. 61.3065. 



Fig. 3. Plethobasus cyphyus (Rafinesque). Small specimen from the Allegheny 

 River, Kelly, Armstrong Co., Pennsylvania, collected September 27, 1907; C. M. Cat. 

 No. 61.3065. 



Fig. 4. Pleurobema obliquum (Lamarck). Normal form of the upper Ohio, 

 diameter 55 percent of length, from the Ohio River, Industry, Beaver Co., Pennsylvania, 

 collected October 3, 1908; C. M. Cat. No. 61.3900. 



Fig. 5. Pleurobema obliquum (Lamarck). Specimen with weak furrow, inclining 

 toward the var. catillus (Conrad), diameter 57 percent of length, from the Ohio River, 

 Industry, Beaver Co., Pennsylvania, collected September 25, 1908; C. M. Cat. No, 

 61.3894. 



Fig. 6. Pleurobema obliquum catillus (Conrad). A specimen closer to catillus 

 than Fig. 5, diameter 56 percent of length, nacre pink. Specimens like those given in 

 figs. 5 and 6 demonstrate that the two forms intergrade. From the Ohio River, In- 

 dustry, Beaver Co., Pennsylvania, collected September 8, 1908; C. M. Cat. No. 61.3895. 



Fig. 7. Pleurobema obliquum rubrum (Rafinesque). Rather typical, diameter 57 

 percent of length, from the Allegheny River, Godfrey, Armstrong Co., Pennsylvania, 

 collected July 13, 1908; C. M. Cat. No. 61.3891. 



Fig. 8. Pleurobema obliquum (Lamarck). Old specimen, drawn out posteriorly 

 and consequently with the low diameter of 42 percent of the length; when young, 

 this was a rather typical obliquum. From the Ohio River, Industry, Beaver Co., Penn- 

 sylvania, collected October 3, 1908; C. M. Cat. No. 61.3900. 



