318 
PALEONTOLOGY OF NEW YORK. 
sure that with our present knowledge of their interior characters, it would 
not be quite as satisfactory to consider them as distinct species. 
We may take a line of section from the Lower Helderberg to the Upper 
Helderberg limestones within the dis nee of two miles north and south, 
and where the conditions of the sediments are nearly similar; and we find 
that the Oriskany sandstone has no representative of the species. The 
Schoharie grit presents the forpi 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 with the Atrypa ( Hipparionyx ) consimilaris of Vanuxem, 
a form occurring in our Corniferous limestone, and which we have generally 
regarded as a variety of Atrypa reticularis. At the same time, Mr. David¬ 
son places Atrypa aspera, 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 in 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 the 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 ven- 
