254 SYSTEMATIC PALEONTOLOGY 



protuberant and projecting beyond the cardinal line; anterior and lateral 

 slopes less abrupt, 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, supported 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, retrorse, with a large chilidium, somewhat concave in 

 the middle. Cardinal process single and erect and, by the coalescence 

 with the divergent, short, crural plates, appearing distinctly trilobate at 

 the posterior end, similarly to P. sericeus, 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. Adductor scars 

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

 shell, slightly divergent, inner margins formed by two ridges, branching 

 from the crural processes and extending to near the anterior margin; 

 outer somewhat indented margin of the muscular impressions greatly 

 elevated as in P. gibbosus Winchell and Schuchert. 



" Dimensions : Length, 8.5 mm. ; width, 10.2 mm. ; height, 5.5 mm."- 

 Euedemann, 1902. 



Occurrence. CHAMBEESBUKG LIMESTONE (Christiania bed). Penn- 

 sylvania, Maryland, and Virginia. 



Kysedorph conglomerate at base of Trenton, New York. 



Collections. Maryland Geological Survey, U. S. National Museum. 



