270 
PALEONTOLOGY OF NEW-YORK. 
This species is intermediate in form and characters between A. naviformis and A. cylindrica 
of the Clinton group, but it is quite distinct from either. It differs from the preceding species 
of this group, in its more elongated form, and the extension of the dorsal valve at the base, 
which character is scarcely conspicuous, or undefined, in the others. Numerous specimens 
having these characters have been observed, all of which are so uniform as to leave no doubt 
of the specific differences between them. 
This species is far less abundant than the others, but has been found usually in isolated 
groups in the calcareous layers of the shale, and not' indiscriminately scattered through the rock. 
Fig. 4 a. Ventral view of a full grown specimen. 
Fig. 4 b. Profile view of the same. 
Fig. 4 c. Front or basal view, showing the extension of the dorsal valve. 
Position and locality. In the Niagara shale at Lockport, usually confined to the thin cal¬ 
careous layers. 
460. 31. ATRYPA RETICULARIS. 
Pl. LV. Fig. 5 a-t. 
For references and synonymes, see page 72 of this volume. 
Shell depressed-globose, or more or less circular, being slightly truncated by the hinge-line ; 
valves nearly equal in size, the ventral valve much more convex than the dorsal valve ; 
dorsal valve often with an undefined depression or sinus at the base, and a corresponding 
elevation on the ventral valve; extremities of the cardinal line sometimes extended a little 
beyond the width of the shell; beak of the dorsal valve small and slightly elevated above the 
ventral valve, or above the hinge-line. Surface marked by from 24 to 30 small rounded plica¬ 
tions, which bifurcate about one-third of the distance from the beak to the margin ; plications 
crossed by prominent imbricating laminae. This description is applicable to the variety oc¬ 
curring in the Niagara group, but not for the species in all its phases. 
This species commences its existence in the Clinton group; and if we include in one 
species all shells having these general characters, it extends through all the succeeding groups 
to the Chemung group inclusive. In each of these positions, however, it possesses some pecu¬ 
liarity by which it may be distinguished ; and the variety from the Niagara group can never 
be mistaken for that from the lower Helderberg limestones, nor the one from the lower for the 
one from the upper Helderberg limestones; nor will either of these be confounded with the 
specimens from the Hamilton and Chemung groups. It is in fact much less difficult to dis¬ 
criminate between specimens of this fossil from the different geological positions, than between 
others which are recognized as distinct species. On this account, I have been inclined to 
recognize them as distinct varieties at least, and there is much foundation for specific distinction. 
Taking the view that these are all of a single species, we must regard it as having possessed 
a remarkable capacity of endurance, and of adaptation to change, to have existed during so 
long a period. Its power of adapting itself to the changed condition is clearly shown in the 
