Contributions to Invertebrate Pal deontology. 511 



The specimens of this species from Ohio are mostly in the con- 

 ditions of internal casts, but a few among them retain the substance 

 of the shell in the condition of a white chalky coating, sufficiently 

 well preserved to afford material for description and illustration. 

 They vary much among themselves in the form of the outline and 

 in the degree of convexity of the valves, a few of them presenting 

 a globular form, while others are but moderately convex. They 

 sufficiently resemble the New York forms to be readily identified 

 where the shell is retained, but in the condition of internal casts 

 are not so easily recognized. The muscular imprints as seen on 

 them are small and faint, those of the dorsal valve narrow and 

 elongated, and that of the ventral is quite small, though deep, and is 

 confined to the rostral portion of the valve. 



Formation and Locality. — In a soft drab-colored hydraulic lime- 

 stone referred to the Lower Helderberg group, at Greenfield, Ohio, 

 associated with forms which appear to represent a Nucleospira. 



Genus IVUCLEOSPIRA Hall. 



Nucleospira rotiiiidata. 



Plate V, figs. 11-14. 



Nucleospira rotundata Wliitf., Ann. N. Y. Acad. Sci., 1882, p. 194. 



Shell attaining a rather large size for the genus, being often more than half 

 an inch in transverse diameter, and when of medium or large size, strongly 

 ventricose or rotund. The younger individuals, however, are depressed- 

 convex or lenticular in profile. Length of the shell as great or greater than 

 the transverse diameter. Beaks small and incurved, not at all conspicuous. 

 Valves marked by a slight depression along the median line, strongest on the 

 ventral side. 



This species, like all those of this formation yet obtained in Ohio, 

 are mostly internal casts and impressions ; consequently the true 

 features of the shell are not readily obtained. The general features 

 of the species, however, are preserved sufficiently for identification 

 and comparison, when good individuals are selected. The shell 

 bears much resemblance to JV^. ventricosa, Con., from the Lower 

 Helderberg group of New York, in its general form, except the 

 much greater size and more elongated form of the adult individuals. 

 There is more difficulty in separating them satisfactorily from the 

 casts of Meristella bella Hall, with which they are associated. In 

 fact, it is all but impossible to do this with certainty, unless they 

 are in a good state of preservation, as the difference in the form of 



