S76 
PALAEONTOLOGY OF NEW-YORK. 
the first third of the length : plications usually bifurcated; the bifur¬ 
cations irregular or unequal. 
The interior of the ventral valve has an elongate spoon-shaped pit, 
the inner extremity of which is free for a considerable extent, and the 
upper part supported on a central septum which usually extends less^- 
than half the length of the shell from the apex. 
In the dorsal valve, the crura or lamellae are conjoined at their bases, 
making a V" shaped trough or pit, which is attached to the valve in its 
upper part and continues sessile for about one-half the length of the 
shell. 
This species, in its A r arious phases of growth and development, presents much 
variety of aspect'and form. In some extreme specimens, it approaches to the form 
of P. knightii, but its prevailing proportions and characters are extremely dif. 
ferent. In young shells the form is sometimes not unlike that of Rhtnchonella ; 
but the greater gibbosity of the ventral valve and the bifurcation of the plications 
offer ready means of distinction. In some of its forms it approaches in character 
the Pentamerelia joapilionensis of the Hamilton group ; but the surface is usually 
marked by a greater number of plications, which are usually more or less angular. 
I have heretofore (Report on State Cabinet, ut cit.) identified the Atrypa arata 
and A. octocostata of Conrad as different phases of this shell, and farther exami¬ 
nation has confirmed this opinion. With a few specimens, each of which might be 
a typical form, it is not difficult to recognize three tolerably distinct varieties of 
this species ; but with a larger number of specimens, they are all shown to belong 
to the same type, and do not offer the means of specific distinction. 
This species, in the collection before me, ranges from half an inch in length 
and breadth to specimens having a length of two inches or mor.e, Avith a width of 
an inch and a half. 
Geological formations and localities. This fossil occurs in the Schoharie grit, 
aud in limestones of the Upper Helderberg group, in Albany and Schoharie 
counties, New-York. It occurs in the same limestone in Cherry valley, at 
Waterville and Babcock’s hill in Oneida county, at Lima in Ontario county, at 
Caledonia in Livingston county, and Leroy and Stafford in Genesee county ; and 
at Clarence holloAv, and five miles east of Buffalo, in Erie county. It is known in 
Canada West, and at the Falls of the Ohio. 
