320 
PALAEONTOLOGY OF NEW YORK. 
Figures 5-7 illustrate the more symmetrical forms of ordinary size. 
The interior of the ventral valve shows a large flabelliform muscular area, which is rounded in front 
and curving to the rostral cavity. Neither in the specimens from this group, nor in those from 
the limestones below, have I observed any thickening of the parts adjacent to the muscular area, 
nor any false area, as in those of the Lower Helderberg group. The rostral cavity is usually large 
and the shell thin; the truncation at the apex is often broad, as if for the protrusion of a strong 
pedicel. This latter feature may be due to accident; but in separated ventral valves it contrasts 
strongly with the usual thickening of the parts about the rostral cavity, as observed in the same 
valves of the Lower Helderberg forms. 
In the dorsal valve, as shown in figure 9, we have a well defined muscular area, divided longitudinally 
by a low septum which becomes thickened in its upper part. The hinge-plate is divided, and pre¬ 
sents on each inner margin a tooth-like plate, between which and the crural bases there is a distinct 
groove. The crura originate close to the apex of the shell and diverge abruptly; the dental sock¬ 
ets are deep in the lower part, and gently curving and narrowing towards , the apex. Outside of 
the socket, and extending along the margin of the shell, is a crenulate callosity, separated from 
the edge of the shell by a narrow groove. 
These features are more fully shown in the enlarged figure 7, of A. impressa, and 
the same, together with the junction of the loop and crura, are shown in the 
accompanying enlarged figure of A. reticularis of the Hamilton group. 
Atrtpa RETICULARIS. 
Interior of the upper part of the dorsal valve, showing the bases of the crura and connecting loop. 
Figures 22 and 23 of Plate 53 a, show the relation of the spires; figure 22 represent these appendages 
lying in the dorsal valve, and connected at their bases by the loop. 
In figure 23 we have the ventral valve mainly removed, showing the dorsal side of the loop and the 
apices of the spires which are in contact. Each cone presents about twenty-two turns of the spi¬ 
ral band. 
In beds referred to the age of the Hamilton group at Independence, Iowa, this species, figs. 14, 15, 
Plate 53, acquires a large size and presents some peculiarities, particularly in the straight extended 
hinge and the suhcarinate dorsal valve. These features have not been observed in specimens from 
the Hamilton group of New York, though simulated by some specimens of the Chemung group 
