218 UNITED STATES GEOLOGICAL SDKVEY OP THE TERRITORIES. 



the other; base forming a subelliptic or semi-ovate curve; dorsum sloping 

 with a concave outline behind the beaks, where it is provided with a dis- 

 tinctly circumscribed, rather broad, lanceolate area ; hinge-margin straight, 

 and notcarinate. Beaks elevated, strongly incurved nearly at right angles to 

 the hinge, closely approximate or touching, located near the anterior extrem- 

 ity. Surface ornamented by small, nearly equal, slightly flexuous, radiating 

 costae, which are crossed by numerous very small, more or less closely- 

 arranged, concentric wrinkles or lines. 



Length, about 2 inches; height, 1.10 inches; convexity, 1.10 inches. 



Near the middle of the flanks, the slender radiating costaa are usually 

 rather more closely arranged, being separated by spaces little, if any, greater 

 than their own breadth. Toward the front, however, where they stand nearly 

 vertical, and on the dorsal and postero-dorsal regions, where they are much 

 more oblique, the intermediate spaces are broader ; while on the anterior 

 extremity, there is an ovate or subcordate area or space, apparently only 

 marked by the concentric wrinkles. The small concentric wrinkles marking 

 the whole surface are usually more closely arranged and less distinct than 

 the costse, to which they impart an obscurely roughened, or minutely tuber- 

 culate appearance. 



It is a neat, symmetrical shell, resembling a Lower Greensand species 

 described by Prof. E. Forbes, in the Jour. Geol. Soc. Lond. (I, 23tS, pi. 2, 

 fig. 3), under the name of P. Martini; but its beaks are more elevated, and 

 placed nearer the anterior end of the valves, and the concentric wrinkles are 

 less distinct, while the radiating costse are smaller and more closely arranged. 



It is also related to a species described by me, from Vancouver's Island, 

 in the Trans. Albany Institute (IV, 42), under the name of P. subelongata ; 

 but it has more elevated beaks, and finer as well as more numerous costse, 

 and is likewise a broader shell, with more obtusely-rounded extremities. 



Locality and position. — Mouth of Judith River, Idaho Territory; in 

 beds now known to hold a position at the horizon of the top of the Fox Hills 

 group. 



