NIAGARA GROUP. 275 
Fig. 1 a, b. Ventral and front views of a young specimen, where the valves are equal and with¬ 
out a sinus. 
Fig. 1 c. A somewhat larger specimen, where the sinus and elevation are slightly developed. 
Fig. 1 d - h. Figures of specimens presenting varieties of form and size, and number of plica¬ 
tions in the sinus. 
Fig. 1 i — Til. Profile views of specimens which are not compressed. 
Fig. 1 n, o, p. Front views showing the elevation of the plications, and the depression of the 
same in the sinus. 
Position and locality. In all localities of the shale of the group from Wolcott to the Niagara 
river, but more particularly at Lockport. 
634. 48. ATRYPA INTERPLICATA. 
Px.. LYII. Fig. 2 a - g. 
Terebratula interplicata. Sowerby : Murchison’s Sil. System, pag. 631, pi. 13, fig. 23. 
Shell spheroidal or subobovate, ventricose in the middle and towards the beaks; ventral 
valve much more convex than the dorsal valve ; beaks very short, closely incurved, the dorsal 
one scarcely so high as the ventral one ; surface plicated ; plications three or four on each 
side at their origin, with an interplication between each pair, except the central ones, com¬ 
mencing about half way from beak to base ; two, three or four central plications on each valve, 
often simple from their origin to the base, and these are depressed on the one and elevated on 
the other valve, forming the mesial sinus and fold, which continue more than half way from 
base to beak j plications crossed by concentric striae and a few imbricating lines of growth. 
The shell described is apparently identical with the species of the Wenlock shale. Our spe¬ 
cimens are often much distorted by pressure; and the great convexity of the ventral valve, 
and its prominence near the beak, causes it, when compressed from below, to project above 
the beak of the dorsal valve. The plications appear as if simple, and the interplications do not 
come from a bifurcation, but rise in the depression between : this gives a peculiar feature to 
the surface, and is a reliable character for distinguishing the shell even when much compressed. 
Nearly all the specimens examined appear to be full grown individuals, and I have had no 
opportunity of tracing it in its various stages of growth. 
Fig. 2 a. Dorsal valve of this species. 
Fig. 2 b. Dorsal view, where the ventral valve near the beak is elevated above the beak of the 
dorsal valve. 
Fig. 2 c, d. Profile views of the specimens 2 a, b. 
Fig. 2 e, f. Front views of two specimens, one of which is compressed. 
Fig. 2 g. An enlargement of several plications, showing the interplications and concentric striae. 
Position and locality. In the shale of this group at Lockport. 
