LOWER HELDERBERG ROCKS. 288 



Fig. 14. Ventral valve, showing the spire on one side displaced. 



Fig. 15. Dorsal valve, showing the spires arranged transversely as in Spirifer. 



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

 group : Base of the Helderberg mountain, Cherryvalley, N. York; and Cumberland 

 in Maryland. 



IVucleospira concentrica ( n. s.). 



Plate XXVIII B. Fia. 15 - 19. 

 Shell depressed orbicular, or sometimes scarcely subrhomboidal in out- 

 line : valves almost equally convex. Ventral valve subangular in the 

 middle towards the beak, sometimes a little depressed towards the 

 front : beak elevated above the opposite valve, acute, incurved, perfo- 

 rate at the apex. Dorsal valve having a central longitudinal depressed 

 line, most convex in the middle and' depressed at the beak, which is 

 closely incurved beneath the opposite one. 

 Surface, when perfect, covered by minute hair-like spines; the shell 

 punctate^ and marked by conspicuous laminse of growth. 

 This species differs from the JV. pisiformis and JV. ventricosa, by having the beak 

 of the ventral valve a little more prominent; while the centre of the shell, from the 

 beak half way to the base, is obtusely angular. The central depressed line, which 

 marks the other species, is sometimes shown towards the base of the ventral valve, 

 while it is a distinguishing feature of the dorsal valve. The elevation of the beak, 

 and the conformity of the cardinal slopes of both valves to this feature, gives an 

 outline to the shell less regularly curved than those mentioned. The concentric 

 laminae are often very regular, and much more conspicuous than on either of the 

 other species observed. The latter feature, together with the elevated subangular 

 beak, are distinguishing characteristics of the shell. 



Fig. 16 a, b, c. Dorsal, ventral, and profile views of a small individual, from which the 

 pilose covering has been removed. 



Fig. 16 d. Enlargement of the dorsal side, showing more distinctly the subrhomboidal form 

 and the concentric laminae of growth. The perforation of the beak appears to 

 have been somewhat enlarged by fracture, but this feature is conspicuous in all 

 the specimens of this species. 



Fig. 17 a, h. Dorsal and profile views of a more gibbous specimen. 



Fig. 18 a. Enlargement of the surface preserving the pilose or fibrous exterior, which in this 

 instance appears to be a thickened fibrous covering of the shell. 



Fig. 18 6. Enlargement of the surface, from which the exterior covering is removed. 

 Fig. 19. A transverse section of a specimen, showing the internal spires. 



Geological position and locality. In shaly limestone of the age of the Lower Helder- 

 berg group : Decatur county, Tennessee. 



