CnONETES OF THE HAMILTON GROUP.' 13a 



deeply pitted outside of the vascular markings, and the margin strongly 

 striate. In other specimens, the marks of striiB and the pitted or punc- 

 tate surface extend over the larger part of the surface, the striae sometimes 

 continuing to the apex. 



The interior of the dorsal valve shows deep dental sockets, with small 

 occlusor muscular imprints and wide vascular areas which are but faintly 

 marked ; while outside of these the surface is strongly pustulose. There 

 is a slender median ridge joining the base of the strong cardinal process : 

 this process is deeply grooved in the middle, both on the inner and the 

 exterior face, and the lateral process on each side is separated from 

 the central one by a similar groove on the exterior or cardinal face. 

 There is considerable difference in the length of this process in different 

 individuals, and it is sometimes constricted below the lateral process; 

 and in other specimens, the parts are equal. The surface is more or less 

 marked by the exterior striae, depending on the thickness of the shell; 

 and not unfrequntly the concentric striae are beautifully preserved upon 

 the interior surface. 



In the character of the surface striae, there are many minute diffe- 

 rences among a large number of individuals. Sometimes an entire surface 

 will be seen to have the striae all bifurcating, or with few intercalated 

 ones ; while others have all or nearly all the increase by intercalation. 



The figures 10 a- g illustrate the principal varieties of form of this species, as it occurs 

 in the calcareous shales of the Hamilton group in Central and Western New- York. 

 Figures 9, 11 and 12 are from the typical specimens of C. tuomeyi, C. martmi and 

 C. madurea of Norwood and Peatten, given for comparison with those of New- 

 York. 



The species described by Mr. Conrad as Strophomena syrtalis (Ice. cit.) seems 

 to me not specifically distinct from C. coronata. It is known in the eastern and 

 central parts of the State, occurring in some harder layers, usually much com- 

 pressed or lying in groat numbers together. The shell is generally partially exfo- 

 liated, or presents the interior surface, giving the character described by Mr. 

 Conrad of " minutely tuberculated ribs." The extreme flattening of the shells, 

 packed together in great numbers, is due mainly to pressure ; and in the same 

 localities, a shell or cast found in softer shale outside of the mass, preserves a 

 moderate convexity. Under these different aspects, it is not surprising that this. 



