LOWER HELDERBERG ROCKS. 
161 
Discing conradi (n. s.). 
Compare Discina (Orbicula) verneuili, Davidson, Bull. Soc. Geol. de France, Deuxieme Serie, 
Tome v, pa. 334, pi. 3, f. 48.- 
Plate IX. Fig. 16, 17 & 17 a; and Plate X A. Fig. 2 a, b. 
Shell orbicular. Dorsal valve very convex, gibbous and obtuse, with the 
apex a little inclined towards the posterior margin, and distant from 
it about one-third the width of the shell. Ventral valve flat or slightly 
concave : foramen submarginal. 
Surface marked by regular distinct radiating striae or ridges, with finer 
concentric striae. 
This species differs from the preceding in the absence of the strong lamellose 
concentric striae, and the stronger radiating striae. The dorsal valve is much more 
convex than in that species; and the ventral valve is concave, and not elevated at 
the apex, while the foramen is near the margin. In single crushed and distorted 
valves there may be sometimes difficulty in distinguishing the species; though there 
are generally some remains of the strong lamellose striae in D. discus , which I 
believe to be always reliable distinguishing characters among the specimens I have 
examined from the localities in New-York. 
In its strong radiating striae, this species resembles the D. verneuili of Davidson, 
cited above; but the striae are represented as coarser and more elevated, and the 
dorsal valve is less convex, while the beak is more nearly marginal. 
The two species compared, viz. D.forbesi and D. verneuili, are from the Wenlock 
limestone of England, a geological series embracing, in different localities, species 
known both in our Niagara and Lower Helderberg groups. 
Fig. 12. Exterior view of the lower valve (improperly represented without the foramen). 
See Plate, X a for corrected illustrations of this and the preceding species. 
Fig. 16. Dorsal valve having the outer shell exfoliated, and showing radiating striae which 
bifurcate towards the margins. 
Fig. 17. Enlargement of the striae. 
Fig. 17 a. Profile view of the dorsal valve. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains, and Becraft’s mountain near Hudson. 
[ Paleontology III.] 
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