318 PALiEONTOLOGY OF NEW YORK. 



sure that with our present knowledge of their interior characters, it would 

 not be quite as satisfactory to consider thorn as distinct species. 



"We may take a line of section from the Lower Helderberg to the Upper 

 Helderberg .limestones within the distance of two miles north and south, 

 and where the conditions of tiie sediments are nearly similar ; and .we find 

 that the Oriskany sandstone has no representative of the species. The 

 Schoharie grit presents the form of A. impressa — but this is distinct from 

 that of the Lower Helderberg limestone, and equally distinct from that of 

 the Upper Helderberg. We have, therefore, three forms which, in collec- 

 tions of hundreds of individuals, do not graduate into each other. 



It is true, that tracing the Upper Helderberg limestone five hundred miles 

 to the southwest, we find the form of Atrypa reticularis changing; but it 

 does not approximate to that of the Lower Helderberg limestone or of the 

 Schoharie grit, nor do any of these forms resemble the one in the Niagara 

 group. 



Several European naturalists, and more recently Mr. Davidson among 

 them (the latter with doubt), have admitted as a distinct species Atrypa 

 desquamata of Sowerby. So far as can be inferred from figures of that spe- 

 cies, it corresponds Avith the Atrypa {Hipparionyx) consimilaris of Vanuxem, 

 a form occurring in our Corniferous limestone, and whijch we have generally 

 regarded as a variety of Atrypa reticularis. At the same time, Mr. David- 

 son places Atrypa asperd, a much more distinct form as I conceive, as a 

 variety of A. reticularis. 



Without attempting to represent more than a small part of the varieties of form 

 observed iu the higher formations, I have endeavored to give a fair expression to 

 those occurring in the Corniferous limestone (Upper Helderberg), Hamilton 

 and Chemung groups. 



The specimens figs. 11-13, Plate LI, from the Corniferous limestone, illustrate the character of the 

 ordinary form and size of this species as it occurs in New York and Canada West. In these forms the 

 ventral valve is often without mesial sinus or depression, as shown in fig. 11, while it is conspicuous in 

 figure 13. The specimens of this general character seldom reach a much larger size in New York than 

 those above referred to, while in the west we have specimens preserving tlie same general aspect and 

 character of striae, with a much more gibbous dorsal valve, and a deep narrow sinus in- the ventral 

 valve. Closely allied to these forms is that represented in figs. 4-6 on plate 52, where the dorsal valve 

 is extremely gibbous, with the anterior portion elevated in the form of a distinct fold, while the vcn- 



