640 GEOLOGY OF OHIO. 



mental area. (Compare plate 45, fig. 8, and plate 47, fig. 1). Eridonychia 

 paucicostata, as the name implies, has fewer costae. E. crenata also has 

 larger costae and is peculiar in having a crenated margin. 



Formation and locality: Middle beds of the Cincinnati group at 

 Cincinnati, Ohio, and localities in the immediate vicinity. Good speci- 

 mens are very rare. 



Eridonychia paucicostata, n. sp. 



Plate 47, Fig-. 2. 



This species, since it agrees very closely with E. apicalis, will be 

 sufficiently characterized by pointing out the only difference of con- 

 sequence shown by the material at hand. The surface namely is more 

 coarsely folded, the costae numbering only seventeen or eighteen, while 

 in that species there are about thirty. It is a difference that strikes the 

 eye at once. 



Formation and locality: I have seen only two specimens. These 

 were collected in the middle beds of the Cincinnati group, at Covington, 

 Kentucky. 



Eridonychia crenata, n. sp, 



Plate 47, Fig. 3. 



This form may be a later variety of E. paucicostata. It also has 

 coarse plications, but there are more of them, the best specimen having 

 twenty-three. A peculiar feature is the projection of the costae at the 

 lower margin, giving a crenate outline. In the other forms the outline 

 is simple. 



Formation and locaity: Upper beds of the Cincinnati group, 

 Waynesville, Ohio. 



Genus ALLONYCHIA, n. gen. 



JMegauibonia, Meek, 1872, Proc. Acad. Nat. Sci. Phila., p. 321. (Not Hall, 1859.) 



Shell attaining a large size, a little obliquely subovate in outline, 

 strongly convex, most gibbous somewhat above and in front of the mid- 

 dle, but with the point of greatest convexity situated, further behind the 

 anterior extremity than in any of the other genera of this family; beaks 

 large, tumid, incurved, not terminal. Hinge line short, not alated pos- 

 teriorly; just beneath the beaks a more or less well-defined, lobe-like pro- 

 tuberance of the anterior side, contains the byssal opening and usually 

 forms the most anterior part of the shell. Surface radially costate. 

 Hinge short, apparently edentulous, ligamental area high; posterior ad- 

 ductor scar large, deeply sinnate above, situated somewhat behind the 



