684 GEOLOGY OF OHIO. 



If the published figures and description of Conrad's species do it justice, 

 the Cincinnati shell under consideration is certainly, distinct. 



The outline is quite different from that of any of the other species 

 of the genus occurring in the Cincinnati rocks. Of Trenton forms cer- 

 tain varieties of L. acuminatum Ulrich, approach it rather closely, but, so 

 far as observed, the post-cardinal angle never projects as far beyond the 

 line of the umbonal ridge as it does in L. subplanum. 



Formation and Locality: Near the base of the Cincinnati group, 

 in, strata exposed in the bank of the Ohio river at Covington, Kentucky- 



Lyrodesma conradi, n. sp. 



Plate 47, Fig. 9. 



Shell a little oblique, transversely subovate, somewhat the highest 

 across the middle of the posterior end; length 15 to 22 mm., height 11.5 

 to 15 mm., thickness about half the height; just beneath the. middle of 

 the slightly oblique posterior margin, the outline is a little produced and 

 more narrowly rounded than elsewhere. Valves moderately convex, the 

 posterior umbonal ridge rounded, not a prominent feature, the beaks small, 

 situated just within the anterior third of the length. Surface marked by 

 very fine, closely arranged, sharp concentric lines, crossed on the pos- 

 terior cardinal slope by about ten radiating striae. Hinge with seven 

 teeth of the usual type in each valve. Adductor scars distinct, the 

 posterior one rather small and situated a very short distance beneath 

 the submarginal pedal muscle impression. Pallial line with a small 

 though undeniable posterior sinus.* A peculiar feature of internal casts 



is the broad and shallow furrow shown in the figure just in front of the 

 umbonal ridge. 



This species resembles L. inornatum but is less broadly rounded 

 posteriorly and has a stronger umbonal ridge. Casts of the exterior or 

 testiferous specimens may be distinguished at once by their surface mark- 

 ings, that species being, as its name indicates, smooth and entirely with- 

 out the posterior rays. The surface markings ally it to L. subplanum 

 and L. cincinnatiense, but the first of these species is longer and quite 

 differently outlined, while in the latter the umbonal ridge is very sharp, 

 the posterior end angular basally, and the beaks more centrally situated. 



Formation and Locality: In the lower 200 feet of the Cincinnati 

 group, at Cincinnati, Ohio, and localities in Kentucky opposite that city. 



*The cast illustrated on plate 47 is a recent acquisition, and since its discovery 

 [ have reexamined the other species accessible to me and found that in every case— 

 they are not many, it must be admitted— where the posterior part of the pallial line 

 could be determined, it was similarly sinuate. 



