324 
PALEONTOLOGY OF NEW YORK. 
cardinal extremities as in the other species. In A. spinosci the beak of the ventral 
valve is shorter, the hinge-line longer and more nearly straight, the muscular area 
proportionally shorter, and the adductor imprint comes down lower and is not so 
clearly pointed. There are also some slight differences in 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 in 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 striae and appfoaches the A. zonata of Schnur (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 costae, 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 costae, the distinction between the two remains as strongly 
marked as at first indicated. 
In the hydraulic 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 New York, there is a form of Atrypa which may be considered 
as intermediate to the A. reticularis and A. spinosa or A. aspera. This form is 
proportionally broader, less gibbous and more 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 graduating into each other.* These are 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 Atrypa aspera or A. prisca of Europe.f It will he observed, 
in the illustrations on Plate li, that the form is somewhat different, the ventral valve more expanded, 
f The form here referred to resembles two large expanded specimens from Refrath in Germany, re¬ 
ceived under the name of Atrypa prisca. 
