STREPTORHYNCHUS. 09 



process between the two branches of the cardinal process ; and below 

 the hinge-line, the flabelliform muscular impression is divided by a low 

 rounded mesial longitudinal ridge or obsolete septum. 



The ordinary size of specimens, in the Corniferous limestone, is about 

 one inch in length by one and a quarter inches in width ; while others 

 measure one inch and an eighth in length, by an inch and a half in width. 

 The largest individual observed is a cast in Schoharie grit, which mea- 

 sures nearly two inches in width. 



I have referred this species to the S. pandora of Billings, with which it cor- 

 responds in general form and character; though the author remarks that "Our 

 " shell closely resembles S. crenistria in external form, but diifers in not having 

 " the radiating striae crenulated, and further in the form of the occlusor muscular 

 " impressions in the dorsal valve. According to Davidson's figure, there is a small 

 " process between the two branches of the cardinal process, which does not exist 

 " in ours." 



The radiating stria;, in all this group of shells that have fallen under my ob- 

 servation, are crenulated by concentric striae'; and in well preserved specimens of 

 this species, these crenulating striae are very distinct. The median process is well 

 preserved in our specimens, and forms one of the slight distinguishing features 

 between it and S. woolworthana ; though I am not disposed to regard this as a 

 constant character, or one of specific importance. 



Externally there is very little difierence between the S. woolworthana of the 

 Lower Helderberg, and the species under consideration in its symmetrical forms. 

 Among the few difierences observable are the slightly depressed apex of the ven- 

 tral valve, giving the greatest elevation a little below that point in the Lower 

 Helderberg species ; while the area is narrower on that valve, and a little wider 

 on the dorsal valve. The striae are perhaps a little finer in S. pandora, but these 

 characters are subject to much variation. In the interior, the muscular impression 

 of the ventral valve of S. woolworthana is proportionally a little larger, and that 

 of the dorsal valve perhaps not so distinctly defined ; but this feature may vary 

 from difference in the thickness of the shells. In the dorsal valve of iS". pandora, the 

 socket-plates are stronger than in the Lower Helderberg species, and are thickened 

 about the base of the cardinal process, projecting beyond the line of the area, 

 while the processes are not (usually) each doubly grooved as in S. woolworthana; 

 and while in that one there is no median process, the Upper Helderberg species 

 preserves that feature very distinctly, often reaching as high as the lateral divisions 

 of the cardinal process. 



