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TWENTIETH REPORT ON THE STATE CABINET. 



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 repre- 

 sented near the base of the crura, do actually unite, forming a loop which 

 connects the spires, as shown in the accompanying figure. 



Atrypa 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 occurs a form with distinct 

 narrow 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 lamellae. Some of the specimens are 

 two and a half inches in diameter, and the volutions of the internal spires 

 vary from twelve to twenty according to the age of the shell. 



The Atrypa aspera, or its representative, in the beds at Independence 

 and Waverly, has the dorsal valve very gibbous, with the ventral valve 

 nearly flat or concave towards the margins. In the higher beds at Rockford 

 there is a form, in which the valves are more nearly equal in convexity, 

 the plications fewer, and the shell has the aspect and character of A. hystrix 

 of the Chemung group in New- York. We are able, therefore, in the rocks 

 of this age, to recognize over a wide area four varieties of form and surface 

 marking which are pretty constant, two of which may be referred to the 

 type of A. aspera and two to that of A. reticularis. 



