15® 
PALEONTOLOGY OF NEW-YORK. 
25 and 26 of this volume, one cannot fail to be impressed with the pre¬ 
vailing greater width on the hinge-line, and consequent modification of 
form in the American Devonian species, as compared with the Stropha- 
losia of the Permian system in Europe. But while the greater part, and 
perhaps all of these, have a narrow cardinal area and small hinge-teeth 
and sockets, I conceive that their exterior character and expression are 
much more like true Productus than are the Permian species of Stropha- 
losia ; and one of the most marked features consists in the conspicuous 
ears with rows of spines, as in Carboniferous Producti; while their 
anterior extension and highly arcuate forms assimilate them much more 
with Productus than with Strophalosia. 
In the Upper Helderberg group, the Productidse of this character 
make their first appearance in the American geological series, and con¬ 
tinue with increasing numbers through the successive formations to the 
Carboniferous limestone. There are a few individuals of two species in the 
Corniferous limestone, and more in numbers of individuals and of other 
species in the Hamilton group ; but it is difficult to find specimens in a 
condition favorable for satisfactory investigation. A few specimens have 
been obtained entirely separated from the rock; but nearly all are 
imbedded, with the ventral valve adhering to the matrix so that the 
hinge and interior structure can rarely be seen ; while the dorsal valves 
almost uniformly adhere by their exterior surface, leaving only their 
interior for examination. 
In the Chemung group, the specimens occur mostly as casts of the 
interior and impressions of the exterior, and these are frequently much 
distorted. In some of the softer compact shales of this group, we find the 
shell partially preserved, but still in such condition that the entire cha¬ 
racters cannot be discovered; and it is only in some fresh exposures of the 
semicalcareous bands that we are able rarely to obtain specimens of more 
natural appearance and better preserved surface. 
Under circumstances of so much difficulty, it is with much hesitation 
