662 GEOLOGY OF OHIO. 



ModiobpsidcE is to be regarded as entirely provisional and mainly in 

 deference to the views of others. While I might indicate a number of 

 differences between Cymatonota and Orthodesma, I deem it best to post 

 pone their consideration till some more final classification of the genus is 

 undertaken. For the present the generic difference of the two groups of 

 species will be sufficiently established by the mention of a single point. 

 Namely, the muscular scars of Orthodesma are precisely as in Modiolopsis, 

 the anterior adductor scar being strongly impressed, while the posterior 

 scar is faint. In Cymatonota however both scars are exceedingly faint, 

 and even in very favorably preserved specimens not the slightest traces 

 of them are to be observed. 



Conrad's species pholadis, which I regard as belonging to Cymatonota, 

 is the first to follow his generic description of Orthonota, and would 

 therefore under ordinary circumstances be entitled to consideration as the 

 type of the genus. It is however clear that Mr. Conrad always regarded 

 his O. undulata as the typical species. 



Cymatonota typicalis, n. sp. 



Plate 55 Figs, 1—5. 



Shell elongate, with the dorsal and ventral margins parallel, the 

 length three and one-half times the height, the greatest thickness, which 

 is a little behind the center, about two-thirds of the height; anterior end 

 nearly vertical, rounded but not uniformly, the turn into the hinge line 

 being rather abrupt; posterior margin rounded, slightly oblique, most 

 prominent in the lower half; ventral margin gently concave. Beaks 

 appressed, scarcely prominent, situated one-fifth of the length of the 

 shell behind the anterior extremit}'; umbonal ridge and mesial sulcus 

 rather distinct features, cardinal region anterior to the beaks sharply com- 

 pressed. Surface with fine equal striae anterior to the beaks, of which 

 not over half continue over the flanks of the shell where they take on an 

 irregular character, some being much stronger than the others; or several 

 may be united into a fold. The umbonal ridge is almost smooth, but the 

 upper part of the posterior cardinal slope is marked with rather regular, 

 strong, oblique folds. 



Formation and Locality: Upper beds of the Cincinnati group. 

 Waynesville, Ohio. 



Cymatonota recta, n. sp. 



Plate 55, Figs. 8 and 9. 



This species is very much like C. typicalis but may be distinguished 

 by the following differences: The length is a trifle greater, the height 

 and length of a specimen in which these dimensions are not affected by 



