682 GEOLOGY OF OHIO. 



polished in the Ctenodonta, while in the second the surface of the casts 

 of the Pyrenomceus are more evenly rounded and the muscular scars 

 much less distinct. 



Formation and Locality: Middle beds of the Cincinnati group. Four 

 specimens only have been seen and these were collected on Mount Au- 

 burn in the city of Cincinnati, from strata lying about 400 feet above the 

 bed of the Ohio river. 



Pyrenomcens szcbciaieatns, n. sp. 



Plate 51. Fig. 6. 



Shell about 11 mm. long, 7 mm. high, and 4.5 mm. thick, cuneate 

 behind, rounded, though not quite regularly, in front; base rounded, car- 

 dinal outline sloping down on each side from the beaks which are situated 

 about one-third of the length behind the anterior extremity. Posterior 

 cardinal slope abrupt. Surface polished, marked with very fine concen- 

 tric striae and a few stronger irregular folds. The specimen figured ex- 

 hibits traces of several radiating lines in the post-cardinal region. 



There are several Lower Silurian shells that resemble the species un- 

 der consideration, but I cannot say that the resemblance is in any case 

 very close. In none of them is the posterior end so cuneate, nor is the 

 surface in any except the preceding species polished. P. decipiens is a 

 higher shell and shorter anterior to the beaks. 



Formation and Locality: Lowest beds of the Cincinnati group, river 

 quarries, Covington, Kentucky. 



Family LYRODESMID^B, Ulrich. 



Genus LYRODESMA, Conrad. 



(Ann. Geol. Rep. N. Y., p. 51 ; 1841.) 



Lyrodesma inomalum, n. sp. 



Plate 50. Figs. 10 and 12. 



Shell not known to exceed 18 mm. in length, transversely subovate, 

 compressed convex, the three dimensions length, height and thickness, 

 respectively to each other as nine and seven is to four. Anterior margin 

 regularly curved, base broadly convex, posterior margin slightly oblique, 

 strongly rounded and most prominent below, gently curved and sloping 

 forward above to the subangular extremity of the short hinge line. 

 Beaks rather small, moderately prominent, situated a little in front of the 

 center; cardinal slope compressed, slightly alate; umbonal ridge verj- 

 moderately developed, the greater part of the surface being rather evenly 

 convex. Surface nearty smooth, without radial lines on the cardinal 

 slope, in the best specimens exhibiting only a small number of obscure 



