184 PALiEONTOLOGY OF NEW- YORK. 



This species is quite distinct from any of those already described. In its mesial 

 sinus, it resembles the P. boydii; but it is usually a much more convex shell, and 

 in all the specimens has fewer spines on the ears, and none upon the middle of 

 the shell. The spines and spiniforous tubercles are likewise of a diiferent charac- 

 ter, while the form and expression of the shell is very different. Occurring only as 

 casts in sandstone, the entire charact3rs cannot be ascertained. 



Geological formation and locality. In sandstone of the age of the Chemung 

 group, at Meadville, Pennsylvania, associated with Spirifer disjunctus and other 

 fossils of the age of the Chemung group. 



Productclla onusta (n. s.). 



PLATE XXVI. 



Shell large, broadly semiclliptical in outline, the length and breadth 

 about as four to five : hinge-line about equalling the width of the 

 shell, the convexity nearly hemispheric. 



Ventral valve regularly convex, depressed-convex in the smaller indi- 

 viduals and ventricose in the older ones, regularly rounded and curv- 

 ing to the front and baso-lateral margins. Body of the shell abruptly 

 curving towards the umbo, which is elevated above the hinge-line, 

 and the apex closely incurved ; abruptly depressed, and concave be- 

 tween the umbo and the cardinal extremities, which are produced in 

 ears comparatively wide, nearly flat, and usually a little rounded at 

 the extremities. / 



Dorsal valve broadly and deeply concave, corresponding to the ventral 

 valve, with the hinge-line comparatively a little shorter. 



Surface of the ventral valve with obscure radiating and concentric striaa 

 and a few wrinkles on the ears, which are partially extended to the 

 body of the shell. These wrinkles of the cardinal margins are studded 

 with the bases of slender spines, and the entire surface is marked by 

 concentric rows of similar but smaller spiniferous tubercles. On the 

 shell, these rise almost directly from the surface without any elonga- 

 tion ; but where the shell is exfoliated, the remaining pustules are 

 elongate. 

 The partially exfoliated ventral valve is peculiarly punctate or pitted, 



as if by the most minute corrugation. The entire cast is puncto-striate, 



presenting an imusual marking. 



