STREPTOEHYNCHUS. 7S 



I am unable to find any characters by which the specimens from the two localities 

 "may be separated into distinct species or even marked varieties. In my collections 

 from Ohio, the specimens of Streptorhynchus are comparatively less common than 

 in similar collections made in the Chemung group of New- York. In the rock still 

 farther to the west, the sandstones of the "Knobs" near New- Albany, I find a 

 Streptorhynchus of similar form and proportions, varying chiefly in the more 

 distinct crenulation of the striae. This feature, however, appears to me oftener due 

 to the nature of the matrix in which the fossil is imbedded, than to any original 

 difference in the shell itself. 



Whether these forms can "be traced into the large species of the upper sand- 

 stones of the Knobs and of the Keokuk formation {Orthis keokuk, which Mr. 

 Davidson has identified with Streptorhynchus {Orthis) crenistria of Europe), re- 

 mains still undetermined. My collection does not furnish intermediate forms suf- 

 ficient to justify such a conclusion at the present time. 



The large specimens from the higher part of the sandstone and the concretio. 

 nary bands of limestone near New-Albany have always a thin median septum in 

 the ventral valve, extending from the apex more than one-third the length of the 

 shell. The cardinal process of the dorsal valve has, moreover, at its base, on one 

 side, a short vertical ridge ; while between these, and immediately below the 

 process, the valve is a little thickened on the inner side. The dental sockets are 

 nearly filled up, leaving a thin accessory ridge on each side of the cardinal pro- 

 cess. The shells have been so thin that the casts show very little muscular marking- 



The illustrations of S. crenistria of Europe, which I have seen, do not presenl 

 the distinct median septum in the ventral valve, which is seen in the large speci- 

 mens referred to. 



Specimens from the Keokuk limestone show a large muscular impression in the 

 ventral valve, which is divided by a longitudinal septum reaching to the base of 

 the imprint. The same feature exists in specimens of all dimensions from the Coal 

 measures. 



