PALAEONTOLOGY 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. 
Platycftras vcntricosum. 
The spine-bearing forms begin their existence in the Oriskany sand- ’ 
stone, are more abundant in the Upper Ilelderberg 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. 
3. 4. 
PLATYCERAS TRILOBATUM. PlATYCEEAS BISULCATUM. PLATYCERAS INTERMEDIUM. 
