158 
PALEONTOLOGY 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 measures inch in length and -f a 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 teelh-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 differences 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 Marcellus 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 pronjinent. 
In comparing a considerable number of specimens of Productus shumardianus from 
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 pyxidatus. The same differences are observable upon the 
dorsal valves of which we usually have the interior surface exposed; sometimes 
few, and sometimes numerous nodes, indicating the fossets of the exterior. 
* In the greater number of individuals examined, it is impossible to determine that teeth-sockets do 
exist; the appearance, even under a lens, indicating their absence. 
