Clinton group. 
75 
silvery lustre. It is easily recognized by its convex, nearly orbicular dorsal and flat ventral 
valve, differing 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 strise less conspicuously than the smaller ventral valve preserved in iron 
ore, where more of the calcareous matter of the shell remains. 
Fig. 10 c. A large dorsal valve, showing the concentric striae 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 /. 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 PLANOCONYEXA (n. sp.). 
Pl. XXIII. Fig. 11 a-A. 
Shell semiorbicular, plano-convex, longer than wide, plicated ; ventral valve nearly flat, and 
often slightly concave; dorsal valve convex, subcarinate near the beak; beak small, promi¬ 
nent, and closely incurved over the ventral valve; surface marked by rounded dichotomous 
plications and concentric imbricating lamellose strise; plications sixteen to twenty-four on 
each valve. 
This species at first view resembles the A . 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 strise 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. lli. 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. 11 c. Dorsal valve of the same specimen, showing the plications more dichotomous. 
Fig. 11 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 f. Profile view of a specimen of ordinary size. 
Fig. 11 g. A cast of the ventral valve. 
Fig. 11 k. The interior of the hinge of a ventral valve. 
