150 FALiEONTOLOGY 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 SxROPnA- 

 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 Pboductus 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 diflScult 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 fo 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 dijBficulty, it is with much hesitation 



