188 PALEONTOLOGY OF NEW JERSEY. 



equal, giving a nearly round section. Anterior umbonal ridge inflated 

 and nearly subangular in some cases, always sharply rounded, and the 

 anterior surface somewhat flattened or but little convex. Central reg'ion 

 of the valves sulcated obliquely, more or less constricting the front margin 

 at about or just behind the center. Hinge-line straight, deeply sunken 

 between the large, inflated, and enrolled approximate beaks. Surface ()f the 

 shell marked by strong radiating ribs, numerous but somewhat irregular 

 posterior to the umbonal angle, but few and distant in front; also by com- 

 paratively strong concentric ridges, which are distinctly deflected at the 

 mesial sulcus and pass obliquely upward in front of it. These concentric 

 ridges form flattened nodes of the radiating ribs by crossing them on the 

 anterior part of the shell. 



I have seen several casts of this species, and notice considerable varia- 

 tion in their characters, especially in the strength of the surface markings, 

 in the form of the anterior end, and in the strength of the mesial sulcus of 

 the valves, and especially in the strength and character of a sometimes 

 deeply impressed but narrow line marking the bottom of the sulcus and 

 dividing the anterior and posterior sections of the shell, it being in some 

 instances almost obsolete. Mr. Morton's t} pe specimen, which I have not 

 seen, seems to have been very small, and to have had the anterior end 

 rounded from below, while Mr. Conrad's type of P. pedorosa is full and 

 round below and sloping above, while a cast of a single valve which is 

 figured; appears to have been quite sharply truncate in front and angular 

 on the umbonal ridge. There is also much diff*erence in the proportional 

 strength of the two sets of ribs in the different examples. 



Formation and locality, — In the Lower Green Marls at Mr. Gr. C. 

 Schanck's marl pits near Marlborough, New Jersey. Mr. Conrad's example, 

 which is the largest and most perfect individual observed, was collected at Tin- 

 ton Falls, Monmouth County, by Prof L. Vanuxem. A single specimen of 

 medium size in the collection of the Academy of Natural Sciences at Phila- 

 delphia, placed with the type of P. pedorosa under the same label, appears^ 

 from its lithological character, to have come from the yellow limestone of 

 the Middle Marl Beds, but may be deceptive in this respect. 



