138 PALEONTOLOGY OF NEW- YORK. 



In the TuUy limestone of New- York, we have a Chonetes which I am unable 

 to distinguish by any positive specific characters from the C. logani of Burlington 

 and other western localities.. 



The shell is moderately concavo-convex, semielleptical, the length 

 and breadth being about as 6 or 7 to 9 ; the cardinal angles scarcely 

 extended beyond the width of the shell. The ventral valve is moderately 

 convex, sometimes gibbous a little above the middle, with the umbo 

 prominent, regularly curving to the front and abruptly depressed towards 

 the cardinal angles. The dorsal valve is regularly and moderately con- 

 cave in the middle and lower part, and flattened towards the cardinal 

 extremities. 



The surface is marked by from twenty to forty low rounded costae, 

 many of which are dichotomous towards the margin. These costas some- 

 times become obsolete towards the cardinal angles, but are often distinct 

 nearly to the hinge-line. The entire surface is marked by elevated undu- 

 lating concentric striae which are sometimes equal to the space between 

 them; but, when partially exfoliated (and sometimes in other condi- 

 tions), are narrower than the intermediate spaces. 



The margin of the cardinal area preserve four (or six ?) somewhat 

 strong oblique spines ; the area is narrow. The interior of the dorsal 

 valve is papillose, with a short cardinal process. Casts of the ventral 

 valve are very convex in the upper part, with a narrow slit marking the 

 place of the median septum, and, below this, preserving the marks of the 

 bifurcating striae, and sometimes the concentric striae are visible near 

 the margin. 



In the shaly and calcareous rocks of Medina county, Ohio, there is a 

 similar form of Chonetes, undistinguishable by any external characters 

 from the specimens from the Tully limestone, except that the cardinal 

 angles are sometimes a little more produced than is usual in that rock. In 

 one specimen, equal in size to the largest from the Tully limestone, there 

 are about forty costae on the margin, and these are crossed by elevated 

 striae of precisely the same character. In numerous casts of the ventral 

 valve, in the same association, there are characters similar to those 

 noted in the Tully limestone specimens ; the central or upper convex 



