«OJI PALEONTOLOGY OF NEW-YORK. 



the middle : mesial elevation rounded, moderately prominent. Foramen 

 narrow, the margins exsert. 



Surface marked by four, five, or six rounded or obtusely angular plica- 

 tions upon each side of the mesial fold and sinus, concentrically 

 marked by fine closely arranged granulose lamellas, which are strongly 

 arched upon the central plications. 

 This species, in its surface markings, resembles S. cyclopiera and S. crispus; but 



the lamellae are more closely arranged, aud the plications are subangular or less 



broadly rounded than in those species : the character and proportion of the area 



and foramen are likewise quite different. 



Fig. 2 a, b, c. Dorsal, ventral, and profile views of the same speciraen. 

 Fig. 2 d, e. Cardinal and front views of the same. 

 Fig. 2y. Enlargement of the surface. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Becraft's mountain near Hudson; and Decatur county, Tennessee. 



Spirlfer teniiistriatus ( n. s.). 



Plate XXVIII. Fig. 3 a - rf. 



Shell subrhomboidal, length and breadth about equal ; cardinal extremi- 

 ties rounded : valves about equally convex : hinge line less than the 

 width of the shell. Ventral valve much longer than the dorsal, greatest 

 convexity nearly opposite the hinge line, much elevated towards the 

 nmbo, with the beak abruptly incurved over the foramen : sinus shal- 

 low, curved above, and becoming flat in the bottom towards the base. 

 Area not strongly defined, high, not exceeding half the width of 

 the shell. Dorsal valve semielliptical, most convex in the middle : 

 mesial fold broad, rounded, prominent towards the front. 



Surface marked by five or six depressed rounded plications upon each 

 side of the mesial fold and sinus, and which become gradually obsolete 

 towards the margin of the shell ; the entire surface covered by ex- 

 tremely fine radiating striaj which are scarcely visible to the naked 

 eye, and these are crossed by finer concentric stria) which crenulate 

 the radiating striae. 



