144 PALEONTOLOGY OF NEW-YORK. 
183. 16. ATRYPA HEMIPLICATA (n. sp.). 
PL. XXXII. Figs. 10 a, b, c, d, e, f. 
Subglobose, pentagonal, wider than long, thickness often equal to the length ; cardinal 
line distinct, short, with (in some specimens) the appearance of a small area on the dorsal 
valve; dorsal valve depressed-convex, with an abrupt, broad, not deep sinus, which 
commences nearly half way from the beak to the base, the beak very small and closely 
incurved ; ventral valve very convex, becoming gibbous, with a broad mesial elevation 
commencing one third of the distance from beak to base, more gibbous towards the beak ; 
sinus marked by two or three strong plications, with three or four upon the mesial lobe 
and two or three upon each side, all of which reach from one third to one half the distance 
from the base to the beak of the shell, leaving the upper half entirely free from these 
markings ; entire surface ornamented by fine, concentric, filiform, subimbricating lines, 
which are more conspicuous towards the base of the shell, and beautifully undulated in 
crossing the plications. 
This peculiar and beautiful species is rarely found in western localities, but is never- 
theless extensively distributed in New-York. It is readily distinguished by its usually 
rotund figure, and the short prominent plications on the lower half of the shell, while it is 
smooth above, or only marked by the fine concentric lines. The sinus at the base usually 
occupies about one fifth of the circumference of the shell, which, with the basal and cardinal 
slopes, gives it a pentagonal appearance. In many specimens, collected from the crystalline 
limestone, I have been able to observe only the plications on the mesial portion ; while in 
the softer or shaly limestone, the lateral ones have all the distinctness of those presented 
in the figures. 
Fig. 10 a. Dorsal valve of a specimen of the ordinary size. 
Fig. 10 b. Profile view of the same. 
Fig. 10 c. Front view of the same, dorsal valve below, showing three plications in the sinus and four 
on the mesial lobe. 
Fig. 10 e. Ventral valve of a large specimen, with only three plications on the medial lobe. 
Fig. 10 f. Front view of the same, showing the extreme gibbosity of the ventral valve. 
Fig. 10 d. Dorsal valve and front view of a young specimen, having but a single plication in the sinus, 
which is only rudimentary, and two on the medial lobe. The two valves in this shell are 
almost equally convex, the beak of the dorsal valve curving very neatly over that of the 
ventral valve. 
Position and locality. In the thin shaly layers near the base of the Trenton limestone at 
Middleville, and in the higher crystalline portions of the same rock at Watertown, Turin, 
and other localities in the northwestern part of New-York. (State Collection.) 
