NOTICE OF PALEONTOLOGY OF NEW YORK. 187 



limestone, Hamilton and Chemung groups. In the Hamilton group it is 

 more abundant and better preserved than in any other formation ; and 

 Gccm-ring in the same beds with A. reticularis, it never approaches that 

 one in character ; there is no difficulty in distinguishing the one from the 

 other ; and the same is true of these forms in the Chemung group. It is 

 also observed that the same distinction between these species exists in 

 Illinois and Iowa. 



The Atrypa hystrix of the Chemung group is likewise regarded as a 

 distinct species, though possessing many features of an extravagant A. 

 sjnnosa. 



A species of Atrypa, closely resembling the A. marginalis of Dalman, 

 occurs in the Corniferous limestone. This species, Atrypa pseudomarginalis, 

 is of rare occurrence in the rocks of New York. 



While this volume has been going through the press, Mr. R. P. Whit- 

 field has made examinations of the 

 internal appendages of several forms 

 of Atrypa, and has found that the 

 short processes, usually represented 

 near the base of the crura, do actu- 

 ally unite, forming a loop which 

 connects the spires, as shown in the accompanying figure of J., reticularis. 



From collections made in Iowa during the geological survey, and from 

 others more recently made, in different places in that State, by Mr. R. P. 

 Whitfield, at points more than a thousand miles west of New York, we 

 learn that in all localities the distinction between Atrypa reticularis, or 

 its representative, and the associated species, is more strongly marked 

 than in the eastern collections, and there is nowhere any indication of a 

 gradation from the one to the other. At Waterloo, in beds which are 

 apparently of the age of the Upper Helderberg group, there occm-s a 

 form with distinct nari'ow plications, a regularly convex dorsal valve, and 

 a flat or concave ventral valve. It is not very unlike a strongly plicated 

 form from Refrath in Germany, or approaching A. insquamosa of Schnur. 



At Independence and Waverly the specimens resemble the finely plicate 

 Atrypa prisca from Refrath, with the margins compressed, the dorsal 

 valve very convex, and the ventral valve flattened or concave towards 

 the margin. They have very conspicuous concentric lamellse. Some 

 of the specimens are two and a half inches in diameter, and the volu- 

 tions of the internal spires vary from twelve to twenty, according to the 

 age of the shell. 



