412 GEOLOGY OF OHIO. 



upper part, approaching gibbous on the umbo, the beak small but strongly incurved; 

 front of the valve truncate or slightly emarginate to accommodate the front exten- 

 sion of the ventral, but no distinctly defined mesial fold exists. Surface of the 

 shell marked only by numerous concentric lines of growth, some of which are 

 strongly defined. 



The specimens of this species noticed from Ohio are smaller than 

 the usual size of individuals from New York, but present the usual 

 features of the species as shown on the specimens figured by Prof. Hall, 

 on Plate XXXIX, Pal. N. Y., vol. 3, fig. 3, f and k, which is by far the 

 most common and characteristic form among those from that State. The 

 Ohio specimens are internal casts, and show the slit in the dorsal valve 

 caused by the removal of the median septum very distinctly. The casts 

 of the ventral side show the characteristic form of muscular impression 

 but it is small and faintly marked. 



Formation and Locality. — In the hydraulic beds of the Lower 

 Helderberg group, at Greenfield, Ohio. 



Meristella bella. 



Plate I, figs. 8-10. 



Merista bella Hall, 10th Rept. State Cab., 1S57, p. 92. 

 Meristella bella Hall, Pal. N. Y., vol. 3, p. 243, pi. xl, fig. 1. 



Shell rather below a medium size, somewhat oblate in form, at least as wide as 

 long, the narrowing of the beak giving an oblate appearance to the shell. Va'.ves 

 usually ventricose and sometimes highly convex, generally a little more full above 

 than below the middle ; margins of the shell regularly curved except near the beak, 

 which is slightly projecting and moderately incurved. Surface of the valves 

 smooth, but each characterized by a slightry impressed mesial sinus along the 

 centre, more strongly marked on the ventral than on the dorsal side, and which not 

 unfrequently causes an emargination of the front border of the shell. 



The specimens of this species from Ohio are mostly in the conditions 

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

 in the condition of a white chalky coating, sufficientry well preserved to 

 afford material for description and illustration. They van- 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 moderate^ convex. They sufficiently resemble the New York forms 

 to be readih 7 identified where the shell is retained, but in condition of in- 

 ternal 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 con- 

 fined to the rostral portion of the valve. 



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

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

 with forms which appear to represent Nticleospira. 



