LOWER HELDERBERG ROCKS. * 161 



Discina conradi ( n. s.). 



Compare Discina (^Orbiculd) vemeuUi, Davidson, Bull. Soo. G^ol. de France, Deuxieme S^rie, 



Tomev, pa. 334, pi. 3, f.48. 



•Plate IX. Fig. 16, 17 & 17 a; and Plate X A. Fig. 2 a, h. 



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 regidar 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 strise. The dors'al 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 difliculty in distinguishing the species; though there 

 are generally some remains of the strong lamellose strise 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 strise, this species resembles the D. verneuili of Davidson, 

 cited above; but the strise 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, me from the Wenlock 

 limestone of England, a geological series embracing, in diiferent localities, species 

 known both in our Niagara and Lower Helderberg groups. 



Pig. 12. Exterior view of the lower valve ( improperly represented witliout 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. 



[ Palaeontology III.] 21 



