692 GEOLOGY OF OHIO. 



Rhytimya compressa, n. sp. 



Plate 56. Fig. 13. 



Shell of medium size, compressed-convex, elongate, the length two 

 and' one half times the greatest height. Beaks small, very little promi- 

 nent, about one-fifth of the entire length behind the anterior extremity; 

 umbonal ridges very inconspicuous; mesial sulcus scarcely distinguish- 

 able. Cardinal margin long, about seven-eighths of the entire length of 

 the shell, straight posterior to the beaks, declining very little anterior to 

 them; anterior end wide, sharply rounded above, uniformly curved in the 

 middle and below; ventral margin gently but almost regularly convex 

 throughout; posterior margin oblique, strongly rounded in the lower 

 half, subtruncate in the upper; post-cardinal extremity obtusely angular. 

 Concentric surface markings as usual for the genus. Radial markings 

 not shown by the specimen figured which is the best seen. From the 

 same block of shale, however, I obtained fragments of the posterior part 

 of a Rhytimya, probably of this species, that are beautifully marked with 

 closely arranged radial series of exceedingly small spines. In a cross- 

 light the lines formed by them are just visible to the unassisted eye. 



The slight convexity of the valves and the absence of a distinct 

 mesial sulcus or constriction sufficiently distinguishes this species from 

 all the other shells referred to Rhytimya save the next, R. radiaiis, 

 which see. 



Formation and Locality: Middle beds of the Cincinnati group, Cin- 

 cinnati, Ohio, about 325 feet above the Ohio river. 



Rhytimya radiata, n. sp. 



Plate 53. Figs. 10-12. 



This species is closely related to R. compressa, but seems to have 

 been more convex, and is a smaller shell. There are slight differences in 

 the outline, the posterior margin being less oblique, the basal line straight 

 in the middle and for a short distance forward, and the anterior end more 

 angular above. Of the surface markings the grano-lineate ornamentation 

 is coarser, and the anterior concentric folds smaller. Finally the length 

 is proportionally a little greater, the height being to the length as three 

 is to eight instead of three to nine. 



The mesial sulcus being very slightly developed the species is 

 readily separated from R. prodticta and R. cehana. From the other species 

 R. radiata differs in such obvious respects that comparisons are not nec- 

 essary. 



Formation and Locality : Lower beds of the Cincinnati group, Cin- 

 cinnati, Ohio, and Covington, Kentucky ; 100 to 150 feet above the bed 

 of the Ohio river. 



