LOWER HELDERBERG ROCKS. 181 



The great variety of surface mai'king, as well as of form and degree of convexity, 

 has caused this shell to be referred to several distinct species. After a careful com- 

 parison of the specimens from the localities cited by Mr. Conrad, and a large col- 

 lection from other places, I am quite unable to discriminate specific distinctions. 

 Indeed so gradual and almost imperceptible is the change from the greatest extremes, 

 that no external characters can be seized upon for description, which are not liable 

 to variation in the next specimen examined. 



The descriptions given by Mr. Conrad are as follows : 



" S. varistriata ( PI. 14, f. 6). Semiorbicular : lower valve ventricose, slightly bent or 

 suddenly rounded toward the base, with prominent sharp radii alternated in size, and 

 the intervening spaces with minute longitudinal lines ; umbo narrowed and convex ; 

 sides towards the hinge flattened ; apex slightly prominent." 



" S. rectilateris ( PI. 14, f. 7). Semioval : lower valve ventricose, with sharp crowded 

 fine radii alternated in size ; dislj, from beak to base, regularly arched or convex ; 

 hinge extremities very little salient, angular ; lateral margins obliquely subrectilinear ; 

 apex prominent." 



" S. impressa. Semiorbicular or semioval : inferior valve ventricose or acutely rounded 

 in the middle ; umbo convex depressed ; apex on a line with the hinge margin ; radii 

 fine, crowded, unequal, on the lower half becoming distinct impressed lines, with 

 intermediate very minute raised radiating lines." 



" LeptcBna indenta. Shell with radiating striae ; intervening spaces waved or indented. 

 Length one inch." 



The surface characters of S. varistriata are represented in the figures 9 and 10 

 of Plate VIII, and in figures 1 - 3 of Plate xvi ; while the surface characters of 

 S. rectilateris are seen in figures 4, 7 and 8 of Plate vin, and figures 4 a - d of 

 Plate XVI. The characters of striae ascribed to S. impressa are only observed when 

 the shell is partially or entirely exfoliated, leaving the impression of the stronger 

 single striae, while the intermediate finer ones appear to be elevated, as seen in 

 figures 5 o - d of Plate xvi. This appearance is deceptive, and due in part to the 

 character of the stone, and the close incorporation of the shell with the matrix. 

 Greater or less ventricosity of the ventral valve accompanies all these varieties of 

 surface. 



The Leptana indenta, cited above, as far as can be ascertained, was founded upon 

 an itodividual of this species where the concentric wrinkles or indentations are 

 stronger than usual, and may be regarded as represented in figures 1 a, b of Plate 

 XVI. This is the only species known to me in this geological position, which pos- 

 sesses the characters described by Mr. Conrad. 



The specimens figured on Plate viii are all from the thinbedded Tentaculite 

 limestone of Schoharie and the Helderberg mountains. 



