69J GEOLOGY OF OHIO. 



margin; anterior end gentry concave in front of the beaks, rounding reg- 

 ularly downward and backward from the obtusely angular antero-cardinal 

 extremity which is but little beneath the line of the hinge; ventral 

 margin distinctly though broadhy sinuate, mostly in front of the middle. 

 Beaks of moderate prominence, incurved, situated about one-fourth of 

 the length of the shell from the anterior extremity. Mesial sulcus dis- 

 tinct, broad, directed obliqueby backward, the part of the shell lying in 

 front of it slightly inflated, while posteriorly the surface of the valves 

 rises very gently into a broadly convex umbonal ridge ; cardinal slope 

 concave, compressed and subalate posteriorly. Concentric surface mark- 

 ings very faint on the post-cardinal third, rather strong and irregular 

 on the anterior and lower side of the umbonal ridge and in the mesial 

 sulcus, becoming finer as they pass over the anterior swelling and finally 

 gathered into strong folds with fine lines between them on the antero- 

 cardinal slope. Radial markings not preserved on the specimen described. 



Greatest length 55 mm.; posterior or greatest height 21 mm.; height 

 at beaks 20 mm.; greatest thickness shown by the specimen described 

 only 6 mm. for each valve. The convexity of the valves has doubtless 

 been reduced b3 r pressure. I estimate the entire normal thickness of the 

 shell at about 15 mm. 



This fine species, which it gives me pleasure to name for the dis- 

 coverer, Mr. George Oeh, a liberal collector and careful student of the 

 fossils of the Cincinnati group, is closely related to R. byrnesi, yet may be 

 distinguished readily hy its much greater size, more distinct mesial sul- 

 cus, and subtruncate posterior margin. R. producta is much smaller and 

 narrower posteriorly. 



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

 cinnati, Ohio, where it was found at an elevation of about 390 ieet above 

 the bed of the Ohio river. 



Rhytimya micklcboroughi, Whitfield. 



Plate 56. Figs. 14 and 15 



Orthodesma mickleboroughi, Whitfield, 187S, Jour. Cin. Soc. Nat. Hist., vol. i, p. 139. 



This species is near R. oehana, but is a trifle longer, more produced 

 posteriorly, less rounded in front, and more convex, especially on the 

 umbonal ridge which is more prominent than in any other species of the 

 genus. 



An examination of the original type of this species shows that it is 

 incomplete at the ends and that the abnormally short form which this im- 

 perfection itself would cause has been emphasized by compression. Whit- 

 field's statement therefore that the valves are "twice as long as high" applies 

 only to imperfect specimens like the one used by him and not to such as 

 have retained the normal relations of those dimensions, in which the 



