CONTRIBUTIONS TO PALEONTOLOGY. 279 



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

 granules are arranged in the direction of the striae and along the grooves 

 of the exterior strise, 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 country, 

 the genus begins its existence in the shales of the Clinton group, where a 

 single species has been found. There are two species known in the Lower 

 Helderberg group, and but a single one in the Oriskany sandstone.* 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 Car- 

 boniferous 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 difiicult 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 inves- 

 tigation, 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 ; 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. lepida), I am able to count six spines on 

 each side ; 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. 



In many specimens the spines can be seen to originate near the inner 

 margin of the area, or along the hinge-line, and can be traced in the 

 substance of the shell in an oblique direction inwards, at first nearly 

 parallel with the margins of the foramen ; and it is only after becoming- 

 free, that they take a vertical or obliquely outward direction. 



* The same occurring in the Lower Helderberg group. 



