376 PALEONTOLOGY 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 various phases of growth and development, presents much 

 variety of aspect and form. In some extreme specimens, it approaches to the form 

 of P. knigMii, but its prevailing proportions and characters are extremely dif. 

 ferent. In young shells the form is sometimes not unlike that of Rhynchonklla ; 

 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 JPerUamerella papilionensis of the Hamilton gi'oup ; but the surface is usually 

 marked by a greater number of plications, which are usually more or less angular. 



I have heretofore (Eeport on State Cabinet, ut cit.) identified the Atrypa arata 

 and A. octocostata of Conead 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, tliey 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 more, with a width of 

 an inch and a half. ^ 



Geological foiinations and localities. This fossil occurs in the Schoharie grit, 

 and in limestones of the Upper Helderberg group, in All)any 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 ; §,nd 

 at Clarence hollow, and five miles east of Buffalo, in Erie county. It is known in 

 Canada West, and at the Falls of the Ohio. 



