64 MEMOIRS OF THE CARNEGIE MUSEUM. 



relationship between the two. However, in the beak-sculpjiure, in the hinge, the 

 color of the nacre, and chiefly in the soft parts, there are important differences, 

 amounting to generic distinctness. The color of the soft parts of P. cooperianus 

 is also rather unique (pale orange, found also in the other species of the genus). 

 Q. pustulosa is, of course, distinguished by the soft parts; but there are also differ- 

 ences in the shell. It is generally smaller, the tubercles are not of the peculiar 

 transverse shape, and the outline of the shell is not so oblique. The color of the 

 epidermis and the presence of broad rays near the beaks also distinguish Q. pustu- 

 losa. 



P. cooperianus is very variable in outline. There are specimens, which are 

 almost round, but generally they are drawn out at the lower posterior end, so 

 as to render the outline ovate and oblique, and in some cases even subtriangular. 

 The beaks are always inclined forwards, but they never are at the most anterior 

 end of the shell. Ver.y oblique specimens approach P. cicatricosus (Say), and 

 individuals corresponding to cicatricoides of Frierson (see above) , may be regarded 

 to a certain degree as intergrades between the two. In sculpture this species 

 also intergrades in the direction of cicatricosus, if the tubercles are more or less 

 restricted to the middle of the shell. However, although there are certain indi- 

 viduals, which are intermediate in the one or the other character, there is no com- 

 plete series of intergrades, and all my specimens may be assigned to the one or the 

 other species. 



Specimens from the Cumberland and Tennessee drainages have the sculpture 

 generally better developed than specimens from the Ohio River. 



Localities represented in the Carnegie Museum: 



Ohio River, Shippingport, Cooks Ferry, and Industry, Beaver Co., Pennsylvania; Clarington, Monroe 

 Co., Ohio; St. Marys, Pleasants Co., West Virginia; Marietta, Washington Co., Ohio (V. Sterki); 

 Parkersburg, Wood Co., West Virginia; Portland, Meigs Co., Ohio. 



Cumberland River, Cloyds Landing, Cumberland Co., Kentucky (B. Walker donor). 



Tennessee River, Kuoxville, and Brabsons Ferry, iinox Co., Tennessee. 



Clinch River, Edgemoor, Anderson Co., Tennessee. 



Distribution and Ecology in. Pennsylvania (See fig. 7) : This is one of the rarest 

 species in Pennsylvania, and exists only in the Ohio River in Beaver and Allegheny 

 Cos., and is even there very scarce. I was able to secure only four specimens, one 

 of which was alive. The latter was found upon a shell-bank, in a steady current, 

 associated with the usual bank-forming species. 



Farther down the Ohio, P. cooperianus becomes more abundant, and it is 

 quite common in the region of St. Marys and Marietta, where it is frequently taken 



