FOSSILS OP THE SILURIAN AND DEVONIAN BOOKS. 33 



apex elevated sub-terminal. Ventral valve flat or concave, the apex excentric ; 

 foramen comparatively large, oval, with margins depressed. The shell itself is 

 very thin. 



The surface is marked by fine, closely arranged concentric striae, which are 

 plainly visible on our specimens, though they are internal casts, and entirely 

 silicified. 



This species closely resembles the D. newberry, from Cuyahoga Falls, Ohio; 

 but that shell is somewhat thicker and stronger, with the apex of the dorsal 

 valve more elevated, and the shell is altogether larger than our specimens. 

 One of the specimens before me measures three lines and a half in length, by 

 three lines in width, and its elevation about one line. 



Formation and Locality. The specimens of this species are adhering to other fossils, a fact which 

 would make their identity with Discina doubtful, if not both valves were found so attached. The speci- 

 men* before me, three in number, occupy the valve of a Spir. oweni, and were found in the Devonian rocks 

 of Clark county, Indiana. 



Discina grandis. VANUXEM. 



Plate III., Figure 3. 



Orbicula grandis, Vanuxem. Geol. Kep. 3d dist 1842. 

 Discinv grandis, Hall. Pal. N. Y., Vol. IV., p. 71867, 

 Discina grandis, H. & W. 24th Reg. Eeport, p. 1871872. 

 Discina grandis, H. & W. 27th Keg. Rep., pi. 91875. 



General form broadly and transversely elliptical ; either plano-convex or 

 concavo-convex. Dorsal valve sometimes extremely elevated ; apex sub-central, 

 a little on one side of the transverse axis. Ventral valve usually moderately 

 concave ; foramen reaching from the center or near the center towards one side, 

 but varying somewhat in different individuals. Surface is marked by fine con- 

 centric striae, crowded near the center, and more distant and sharply elevated 

 towards the margin. This species is easily recognized by its larger size, and 

 in the ventral valve by the direction of the foramen being in the shorter diam- 

 eter of the shell. This valve is somewhat unequally concave, and, on the side 

 of the foramen, often a little convex. The specimen here described and figured, 

 has diameters of fourteen and twelve lines, and a height of six lines and one- 

 half, it may be considered as of average size among the shells found here. 



Formation and Locality. This species is often met with in our rocks, but only the dorsal valve Is 

 preserved, it is always entirely silicified, and occurs in the cherty layers, superimposed upon the hydraulic 

 limestone of the Devonian formation, in Jefferson county, Kentucky, and in Clark county, Indiana. All 

 my specimensof this species I found in the cement quarries at Watson's Station, on the Ohio and Missis- 

 sippi Railroad, where the chert beds come near the surface. 

 GEOL. SUB. 5 



