210 



valve and ventricose upper portion of the shell which is so 

 characteristic of the New York species which has been re- 

 ferred to 0. hybrida. 



GENUS STREPTORHYNCHUS (King). 



Streptorhynchus tenuis {Hall). 



Shell large, semicircular or broadly semi-elliptical, cardinal 

 line less than the greatest width of the shell, cardinal ex- 

 tremities rounded. Yentral valve slightly concave ; area 

 narrow ; beak slightly elevated. Dorsal valve moderately 

 convex, umbo not prominent, arcuate near the front 

 margin, and compressed near the cardinal extremities. 



Surface marked by moderately fine, rounded, alternately 

 large and small, threadlike striae, which are strongly curved 

 on the lateral portions of the shell, crossed by very fine 

 concentric stria?, giving under a lens, a beautiful rugose 

 character. Substance of shell very thin. Length of one 

 specimen one inch and a half with a breadth of two inches. 



This species differs from any other of the genus, known 

 to me in rocks of this age, in the rugose surface and round- 

 ed cardinal extremities ; in this respect it has more the char- 

 acter of the Carboniferous species of Streptorhynchus. But 

 few individuals of the species have been observed in the 

 extensive collections made at the locality. 



GENUS STROPHOMENA (Raf.) 



Strophomena (strophodonta ?) SEMIFASCIATA. 



Shell large, transversely semi-circular, width nearly twice 

 the length ; cardinal extremities somewhat rounded. 

 Yentral valve with slightly elevated umbo and small 

 pointed beak ; the middle of the shell is deeply concave, 

 and abruptly inflected towards the front ; area narrow, 

 with a distinct triangular callosity filling the greater 



