158 PALAEONTOLOGY OF NEW- YORK. 



The dorsal valve is marked by few or many interrupted radiating 

 ridges, but no spines have been observed attached to them. 

 The specimens are usually from half an inch to six-tenths of an inch 

 wide ; one well-formed specimen measured j\- inch in length and |i inch 



in width. 



The interior of the dorsal valve is often pretty regularly convex^ and 

 sometimes nearly flat in the upper part and "abruptly deflected towards 

 the front. The surface is marked by interrupted ridges and fine concen- 

 tric striae, with a bilobed or bifurcate cardinal process and faint indica- 

 tions of teeth-sockets.* The muscular impressions have not been dis- 

 tinctly observed ; but the reniform vascular impressions in the Iowa 

 specimens are similar to others of the genus. The interior of the ventral 

 valve is unknown. 



After a careful comparison of specimens from the Corniferous limestone and 

 Hamilton group, with those from Burlington in Iowa and the town of Louisiana 

 in Missouri, I am unable to find any difiercnces which I believe to be of specific 

 importance ; and am therefore compelled to regard those of New-York as identi- 

 cal with those of the West. A specimen from the Corniferous limestone of Ohio is 

 scarcely distinguishable by any character of form, or even in the color of the rock, 

 from specimens of the Oolite limestone at Burlington in Iowa ; and a well-formed 

 specimen (figure 35, Plate 23) from the limestone of the Marccllus shale is dis- 

 tinguished from the Iowa specimens only by its dark color. In each, the surface 

 striae and the ridges at the base of the spines, as well as the form and measure- 

 ments, correspond. It may be observed, however, that the prevailing form of the 

 specimens from Burlington is somewhat broader than those from the Corniferous 

 limestone, and that in the latter the ears as well as the spiniferous ridges are 

 often more prominent. 



In comparing a considerable number of specimens of Productus shumardianus (rom 

 Burlington, Iowa, I find the form of the shell pretty constant ; but there are 

 sometimes more or fewer spines on the umbo, while in some specimens this part of 

 the shell has scarcely any spines. There is a similar irregularity in the distribu- 

 tion of spines upon the body of the shell, and not unfrequently we find it almost 

 destitute of these appendages, and scarcely separable from the forms I have de- 

 scribed as Productus pyridatus. The same difiercnces are observable upon the 

 dorsal valves of which we usually have tho interior surface exposed ; sometimes 

 few, and sometimes numerous nodes, indicating the fossets of the exterior. 



* In the gT«ftter mimber of Individuals examined, it is impossible to determine that teeth-sockets do 

 exist; the appearance, even under a lens, indicating their absence. 



