LOWER SILURIAN LAMELLrBRANCHS. C83 



concentric lines of growth. Eight transversely striated and radially ar- 

 ranged cardinal teeth in each valve, the anterior one usually not quite 

 distinct from the dorsal edge of the valves. Muscular and pallial im- 

 pressions not observed. I i 



This species agrees with L. planum Conrad, in the number of its 

 cardinal teeth and in wanting the post-cardinal striae which are usually 

 present in species of the genus. They are however readily separated by 

 differences in their outlines, L inortiatum being shorter (relatively higher) 

 and more nearly ovate. Hall's L. cincinnatiense presents considerable 

 resemblance in the way of outline, yet is really quite a distinct species, - 

 having a shorter hinge line, only six cardinal teeth, an angular umbonal 

 ridge, and distinctly striated' post-cardinal slopes. 



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

 ington, Kentucky, and Cincinnati, Ohio. 



Lyrodesma grande, n. sp. 



Plate ,50. Fig. 18. 



Of this large species I have seen only the cast of the interior figured 

 on plate 50. While I cannot doubt that it really belongs to Lyrodesma, 

 it is still a fact that there are features in which it differs from all the pre- 

 viously known forms of the genus. In the first place it is much larger 

 than any of the others, and unusually short, while the plate bearing the 

 cardinal teeth seems to have filled the entire rostral cavity, as that practi- 

 cally no beaks are to- be distinguished on the cast. The muscular scars 

 also exhibit peculiarities, the anterior one being very elongate, and the 

 posterior one of unusually large size. Having no strong umbonal ridge 

 ( the surface of the cast is almost uniformly compressed^convex) the affin- 

 ities of the species seen to be nearest L. inortiatum. Compared with that 

 species it is found to differ in having the anterior end relatively narrower, 

 the basal margin more obliquely rounded, and the hinge line shorter and 

 not alate posteriorly. From L. cincinnatiense Hall, it is sufficiently dis- 

 tinguished by its great size and in wanting a distinct umbonal ridge. 



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

 cinnati, Ohio. 



Lyrodesma subplanum, n. sp. 



Plate 47. Kig. 8. 



Compare Lyrodesma planum, Conrad, 1841, Ann. Geol. Rep. N. Y., p. 51. 



The outline of this shell is very much like that of L. planum Con- 

 rad, as figured by Hall in 1847 (Pal. N. Y., vol. i, pi. 82, figs. 11a, 11*). 

 As represented by Hall, that species is longer, without radiating lines on 

 the post-cardinal slope, and with the umbonal ridge not nearly so strong. 



