20 NEW YORK STATE MUSEUM 



near the margins turning suddenly into a natter border. Cardinal 

 area moderately elevated, concave, delthyrium large, of equal 

 width and length; no deltidium observed. Teeth small, sup- 

 ported by strong, diverging dental lamellae, which continue in 

 outward direction into the much elevated margin of the diductor 

 muscles; this margin extends about one fourth the length of the 

 valve, and then returns under an acute angle including a very 

 deep pyriform muscle pit. The muscle margins are separated 

 by a distinct septum, which extends to near the anterior margin : 

 from the anterior part of the -muscular impressions extend 

 strongly marked vascular trunks which are tri- or quadripartite 

 and inclose between them a narrow elongate depressed area. 

 Brachial valve concave in the middle part, closely following the 

 curvature of the pedicle valve, with a well defined ridge all 

 around the lateral and anterior margin. Cardinal area as high 

 as that of the pedicle valve and also slightly concave, retronse, 

 with a large chilidium, somewhat, concave in the middle. Cardi- 

 nal process single and erect and, by its coalescence with the 

 divergent, short, crural plates, appearing distinctly trilobate at 

 its posterior end, similarly to P. s e r i c e u s, with the difference, 

 however, that the posterior ends of the crural plates are not 

 closely appressed to the cardinal process, but separate again 

 a little, forming processes almost as prominent as and parallel 

 to the cardinal process (pi. 1, fig. 18). Adductor scars shallow, 

 broadly triangular, extending not quite to the middle of the shell, 

 slightly divergent, inner margins formed by two ridges, branch- 

 ing from the crural processes and extending to near the anterior 

 margin; outer somewhat indented margin of the muscular 

 impressions greatly elevated as in P. gibbosu s Winchell and 

 Schuchert. 



Dimensions. Length 8.5 mm, width 10.2 mm, hight 5.5 mm. 



Horizon and locality. Extremely common in the pebbles of 

 compact black limestone in the conglomerates of Kysedorph hill 

 and Moordener kill, rare in the matrix and very rare in the gray 

 crystalline limestone pebbles. (Groups 5, 7 and matrix) 



