52 
PALAEONTOLOGY OF NEW-YORK. 
Ortliis cyclas. 
PLATE VII. 
Orthis cyclas : Hall, Thirteenth Report on the State Cabinet, 1860, p. 78. 
Shell small, varying from subcircular to transversely subelliptical, mo¬ 
derately convex : beaks appressed, not distant; cardinal line rather 
less than one-half the greatest width of the shell. Ventral valve convex, 
most gibbous near the umbo : beak small, slightly incurved; area ra¬ 
ther low. Dorsal valve the less convex, sometimes marked by a shallow 
depression : beak very small, slightly projecting beyond the cardinal 
line; area narrow. 
Surface marked by strong sharp prominent striae, which are both bi¬ 
furcated and implanted, often appearing fasciculate near the margin 
of the shell. 
The largest specimen of this species which I have seen is a little more than 
half an inch in length, and aboye six-tenths of an inch in width in its greatest 
diameter. The length of the area is about one-third of an inch. The distinguishing 
features of this shell are its usually nearly circular form, the sharply prominent 
striae, and the comparatively great length of the cardinal line. 
Geological formation and locality. In the shales of the Hamilton group at York 
in Livingston county; Pavilion, Genesee county; near Bellona; and on Canan¬ 
daigua lake in Ontario county, N.Y. 
• Ortliis idonens (n. s.). 
PLATE VII. 
Shell subcircular, a little wider than long, both valves convex and nearly 
equal in length : hinge-line a little more than one-third the greatest 
width of the shell; the extremities rounded into the general contour 
of the shell, which is a little oblate. The dorsal valve is regularly 
convex above the middle, with a scarcely perceptible flattening along 
the median line, and becoming flattened below : beak small; not in¬ 
curved ; area flat, about half as wide as that of the ventral valve. 
