824 PALEONTOLOGY OF NEW YORK. 



cardinal extremities as iu the other species. In A. spinosa the beak of the ventral 

 valve is shoiler, the hinge-line longer and more nearly straight, the musculai* area 

 proportionally shorter, and the adductor imprint comes down lower and is not so 

 clearly pointed. There are also some slight differences iu the interior of the dor. 

 sal valve ; but the specimens examined have been too few and imperfect to render 

 the result satisfactory. 



In the character of the internal spires the distinction is quite palpable : the 

 junction of the crura with the valve differs iu a small degree, as does the loop and 

 its connection with the crura ; while we have about fifteen turns of the spire 

 ■where there are twenty-two in A. reticularis of the same size. The connecting 

 loop does not descend so deeply into the cavity of the valve ; and in its junction 

 with the crura, as well as the form of the latter, it differs from A. reticularis. 



In pursuing investigations to the westward, the contrast between this species 

 and A. reticularis, or its representative, continues to be equally or even more 

 strongly marked. In specimens from Iowa, the ribs of A. spinosa or aspera are 

 stronger and coarser than in specimens from New York ; while the form referred 

 to A. reticularis has finer strias and approaches the A. zonata of Sohnuk (loc. cit). 

 In collections from the Hamilton group near Cumberland (Md.) and the adjacent 

 parts of Virginia, there are many casts and exfoliated shells of A. spinosa, but 

 none of them with the finer costro, or that can be referred to A. reticularis. Although 

 in these species from different localities there Is a palpable variation in the number 

 and character of the costse, the distinction between the two remains as strongly 

 marked as at first indicated. 



In the hj'draulic limestone beds, which lie mainly above the coral-bearing beds 

 at the Falls of the Ohio; at Columbus and other localities in the State of Ohio, as 

 well as in Western Now York, there is a form of Atbypa wliich may be considered 

 as intermediate to the A. reticularis and A. spinosa or A. aspera. This form is 

 proportionally broader, less gibbous and moro strongly plicated than those which 

 we usually refer to A. reticularis — but we do not, in any locality, so far as I know, 

 find these varieties gi-aduating into each other.* These arc illustrated on Plate 

 51, figures 10-24., 



• I am not by any means satisfied that this variety may not prove a distinct species, or it may cor- 

 respond with some of the forms termed jitrypa aspera or A. prisca of Europe.f It will be observed, 

 in the illustrations on Plate Lt, that the form is somewhat different, the ventral valve more expanded, 



t The form here referred to resembles two large expanded specimens from Refrath in Germany, re- 

 ceived under the name of Jjlrypa prisca. 



