320 PALEONTOLOGY OF NEW YORK. 



Figures 5-7 illustrate the more symmetrical forms of ordinary size. 



The iaterior of the ventral valve shows a large flabclliform muscular area, which is rounded in front 

 and curving to the rostral cavity. Neither in the specimens from this group, nor in those frtim 

 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 Holderberg 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 margifi 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. 



AtBTFA BBTICITI.ARIS. 



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, 

 Plato 53, acquires a large size and presents some peculiarities, particularly in the straight extended 

 hinge and the subcarinate 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 



