.VJ PALEONTOLOGY OF NEW-YORK. 



Oillil^i cyclas. 



PLATE Vn. 

 Orthu cyclat : Hail, Thirteenth Report on the State Cabinet, 1860, p. 78. 



Shell small, var3'ing 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 neaf 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 above 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 

 striie, 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. 



Oitlii.s idoneu!^ ( n. s.). 



PLATE VIL 

 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. 



