318 : Cincinnati Society of Natural History. 
descending to the base, gradually widening and deepening, separates 
the anterior from the posterior umbonal region. Surface marked with 
numerous imbricating lines or concentric coste. 
The measurement of three casts is as follows: 
1. Length, 1:95 inches; height, 1-20 inches; thickness, 9-10ths inch: 
2. Length, 1:70 inches; height, 1:05 inches; thickness, 75-100ths inch. 
3. Length, 1:30 inches; height, 80-100ths inch; thickness 65-100ths 
inch. . 
This species is distinguished from C. miamiensis by its elliptical 
shape, projecting anterior end, more ventricose posterior umbonal re- 
gion, less ventricose anterior umbonal region, curving cardinal line, less — 
prominent beaks and other characters, so that the two species may be 
recognized at a glance. It is distinguished from C. neglecta by its 
more elliptical shape, better defined sulcus, more ventricose and less 
angular posterior umbonal slope, and other characters that enable us 
to distinguish the species even from the most imperfect casts. 
Hall and Whitfield described C. miamideénsis as a small shell, but I 
have a specimen of it much larger than the largest illustrated speci- 
men of C. elliptica, and others as small as those which they described. 
This species is from my own collection, and was found in the Hudson 
River Group, near the top of the hills, at Cincinnati. 
PYANOMYA, n. gen. 
[Hty.—Pyanos, a bean; Mya, a genus of shells. ] 
Small, inequilateral, bivalve shells, with thin, fragile, ventricose, 
edentulous valves. Valves united by an external a No 
escutcheon. Type, Pyanomya gibbosa. 
This genus is proposed for edentulous shells found in the Hudson 
River Group without well-marked muscular impressions, or scars pre- 
served on the casts, and which have heretofore been undescribed. 
PYANOMYA GIBBOSA, 0. Sp. 
Plate VIIL., fig, 4, view of the left side, natural size; fig. 4a, view ofthe right side; 
fig. 4b, cardinal view. 
Shell small, very inequilateral, subovoid in outline, with the larger 
end posterior; valves very ventricose or gibbous in the middle or pos- 
terior umbonal region; edentulus; beaks very small, and scarcely pro- 
jecting beyond the cardinal line; ligament, external; hinge line, arcuate 
posteriorly, and strongly bent down in front of the beaks; no escutch- 
eon; lunule, not distinguished; anterior end projecting and sharply 
rounded: into the semi-elliptical base; posterior end more broadly 
rounded; umbonal region, swelling posteriorly, and rounding off in the 
