310 



PAL2E0NT0L0GY OF NEW- YORK. 



their earlier stages of growth, become more or less plicated towards the 

 margin, with several sinuosities in the peristome at the mature condition : 

 usually, however, one or two of the marginal sinuosities are deeper than 

 the others. 



Platyccras ventrico.siim. 



The spine-bearing forms begin their existence in the Oriskany sand- 

 stone, are more abundant in the Upper Helderberg limestone, and one or 

 two are known in the Hamilton group. There are likewise a few spine- 

 bearing forms in the Carboniferous limestone of the West. 



Plattcirai trilobatcv. 



Plattceras bisulcatum. 



PlATYCKBAS INTKKHIflllUM. 



