116 PALEONTOLOGY OF NEW-YORK. 



The interior of the shells are strongly pustulose or papillose : the 

 little granules are arranged in the direction of the strioo and along the 

 grooves of the exterior striae, which often become defined elevations 

 upon the inner surface of the shell. 



So far as we at present know the species in the rocks of this coun- 

 try, the genus begins its existence in the shales of the Clinton group, 

 where a single species has been found. There is no species known in the 

 Lower Helderberg group, and but a single one in the Oriskany sand- 

 stone. Two species are known in the Schoharie grit, and three others 

 are common in the Corniferous limestone ; while others are added to the 

 number in the Hamilton and Chemung groups. Several species are known 

 in the Carboniferous limestones and in the Coal measures. 



For the most part the species are of small size, and, when well pre- 

 served, it is not very difficult to distinguish them ; but in their various 

 conditions of preservation, such as partially exfoliated and more or less 

 worn shells, casts of the interior and impressions of the exterior, in a 

 matrix varying from pure limestone to calcareous shale, and through the 

 arenaceous shales to a nearly pure sandstone, the specific relations often 

 become difficult of determination. 



In a very large proportion of the specimens coming before me for 

 investigation, the cardinal spines cannot be seen, or are but imperfectly 

 preserved. Their comparative strength or direction often furnishes means 

 for specific distinction ; but I have not been able to satisfy myself that 

 the number of spines on the hinge-line is of specific importance, for 

 these are sometimes unequal on the two sides of the apex, and their 

 development seems often to be promoted or retarded by the irregular 

 growth of the shell. The number of these appendages does not depend on 

 the size of the shell; some of the larger species having only three or 

 four spines on each side of the beak, while in the smallest specimens of 

 the group in our rocks (C. lepid'a), I am able to count six spines on each 

 Bide ; and in C. scitula, as many as six or seven, the bases only of many 

 of them being preserved in the substance of the shell as tubular 

 openings. 



