314 
PALAEONTOLOGY OF NEW YORK. 
The muscular area in the bottom of the dorsal valve is usually divided 
by a median ridge; and in old shells of some of the species, this area 
becomes very strongly defined. 
The vascular impressions, in both valves, originate on each side near the 
base of the muscular area, and divide into two principal trunks which 
diverge towards the hinge-line and the front of the shell, and give off 
numerous lateral bifurcating branches. 
The prevailing and conspicuous forms of this genus have been generally referred 
to two species, or rather to a single species, the Atrypa reticularis of LiNNiEus and 
its varieties; though more recently the A. • desquamata of Sowerby has been 
doubtfully admitted as a distinct species. ' My own investigations do not lead to 
the adoption of this view, though I admit that there are many difficulties in the 
way of specific determination, especially where the shells are exfoliated or com¬ 
pressed ; but the examination of large numbers of individuals, together with the 
interiors of many of them, has compelled me to recognize as distinct species some 
of those which are usually considered as varieties.only. How far the complete 
study of the interior structure, and of the spires, will sustain the separation of 
species, remains yet to be determined. 
In order for a satisfactory solution of all these difficulties, a series of observa¬ 
tions should be made upon extensive collections of individuals from widely separated 
localities iu the same geological horizon, and also from different geological forma¬ 
tions, to learn the effect of geographical and chronological influences. I conceive 
that until such investigations shall have been made, we are not prepared for a 
final determination of the questions of specific difference or identity. 
Two species of this genus are of common occurrence in the Niagara and one in 
the Lower Helderberg group; and though I have referred the more common form 
to A. reticularis , there is usually no difficulty in recognizing the specimens from 
each formation, or in distinguishing them from those of the Upper Helderberg 
and Hamilton groups. The same observations are almost equally true of those in 
the higher rocks; but at the same time we find strongly marked features of dis¬ 
tinction iu individuals from widely separated localities of the same formation. 
