silvery lustre. It is easily recognized by its convex, nearly orbicular dorsal and flat ventral 

 valve, JiiTering very widely from every species in the group except the following one, from 

 which it differs in the smaller number and simpler character of its plications. 



Fig. 10 a, b. Dorsal and ventral valve of young specimens. The dorsal valve in shale shows the 

 concentric striee less conspicuously than the smaller ventral valve preserved in iron 

 ore, vi'here more of the calcareous matter of the shell remains. 



Fig. 10 c. A large dorsal valve, showing the concentric strite near the base. 



Fig. 10 d. A group of individuals on a fragment of stone. 



Fig. 10 e. A cast of the ventral valve. 



Fig. 10 f. Several plications enlarged. 



Fig. 10 g. Profile of the elevation of the plications. 



Position and locality. In the upper green shale at Rochester, very abundant ; also less 

 common near the Ridge road in Ontario ; in the iron ore beds at the same place ; at Cental's 

 mill, and Sodus and Wolcott in Wayne county. It likewise occurs more rarely in localities 



west of Rochester. (State Collection.) 



463. 34. ATRYPA PLANOCONVEXA (n. s/>.). 



PL. XXIII. Fig. 11 o- A. 



Shell semiorbicular, plano-convex, longer than wide, plicated ; ventral valve nearly flat, and 

 often slightly concave ; dorsal valve convex, subcarinate near the beak ; becJc small, promi- 

 nent, and closely incurved over the ventral valve ; surface marked by rounded dichotomous 

 plications and concentric imbricating lamellose striae j plications sixteen to twenty-four on 

 each valve. 



This species at first view resembles the ./?. hemispherical but it differs very essentially on 

 closer comparison : it is proportionally less wide ; and the plications, instead of being simple, 

 are dichotomous, more numerous, and rounded. The concentric striee give the plications a 

 rugose character, not noticed in A. hemispherica. The shell is also often marked by lines of 

 growth near the base, which give it a rough and thickened appearance ; which character has 

 not been noticed in A. hemispherica. 



Fig. 11 a. A young individual. 



Fig. Hi. Ventral valve, with the plications nearly all simple. The small neat beak is distinctly 



shown, projecting above the ventral valve, with the foramen open to the extremity. 

 Fig. lie. Dorsal valve of the same specimen, showing the plications more dichotomous. 

 Fig. \\ d. The dorsal valve, having a few of the plications in the middle larger than the others, 



and a small interstitial one in the centre. 

 Fig. 11 e. The dorsal valve of another specimen, in which all the plications are dichotomous, 



numbering twenty-six below the centre. 

 Fig. 11/. Profile view of a specim«n of ordinary size. 

 Fig. 11 g. A cast of the ventral valve. 

 Fig. W h. The interior of the hinge of a ventral valve. 



