14 Cincinnati Society of Natural History. 



Height of a specimen of the usual size, 0.02 inch; breadth, 0.04 inch. 

 This species differs from both C. minuta and C. parvula, Hall, in 



having a larger umbilicus, the spire much more depressed, and the 



whorls angular instead of rounded. 



Formation and locality: found in great abundance in the Hudson 



River Group at Hamilton, Ohio. 



Genus Zygospira (Hall). 

 Zygospira conoentrica, n. sp. (Plate VII., figs, 10, 10a, 106.) 



[Ety.— Concentricua, concentric] 



Shell small, depressed, sub-equivalve, generally a little wider than 

 long; posterior lateral margins straightened, and converging to the 

 beaks at an obtuse angle; lateral margins rounded; front rounded or 

 sometimes a little straightened. Dorsal valve with a shallow, undefined 

 mesial sinus of moderate breadth at the front, but becomes obsolete be- 

 fore reaching the umbo; surface on the sides of the sinus gently convex, 

 and sloping to the lateral margins; beak rather prominent and slightly 

 incurved. 



Ventral valve with a low mesial ridge, most prominent near the middle 

 of the shell, on each side of the ridge the slopes are somewhat depressed ; 

 l)eak small, pointed, projecting be3'ond that of the other valve, and 

 strongly incurved; foraminal aperture very small, round, and situated 

 just under the apex. 



Surface ornamented bj' rather distant but well-defined stria' of growth : 

 sometimes 6 or 8 very obscure radiating folds or plications are ob- 

 served; in that case two occupy the mesial ridge and one the sinus. 



Length of medium-sized specimen, 0.18 inch; width, 0.2 inch; great- 

 est convexity, 0.12 inch. 



The outlines of Zygospira modesta, Say, are quite similar to those of 

 this shell; that species has, however, from 16 to 20 strong and angular 

 radiating ridges, and only very rarely has the fine and crowded concen- 

 tric lines preserved; but in this species there are generally no radiating 

 plications (when any do exist, they are only rudimentary), while the 

 concentric striic are well developed; besides, the posterior lateral mar- 

 gins are straighter, and the beak of the ventral valve is more pointed, 

 than in that shell. 



Formation and locality: found in the lower part of the Hudson 

 River Group, on the hills about Cincinnati, at an elevation of from 300 

 to 350 feet above low water mark. 



Collectors: W. Gault, W. E. Cook, E. O. Ulrich. 



