LOWER HELDERBERG ROCKS. 
223 
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 Helclerberg 
group : Base of the Helclerberg mountain, Cherryvalley, N. York; and Cumberland 
in Maryland. 
IVudeos])ira conceiitrica (n.s ). 
Plate XXVIII B. Fig. 15 - 19. 
Shell depressed orbicular, or sometimes scarcely subrliomboiclal 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 laminae of growth. 
This species differs from the JY. pisiformis and JY. 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 
laminse 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 «, b, c. Dorsal, ventral, and profile views of a small individual, from which the 
pilose covering has been removed. 
Fig. 16 (1. 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, b. 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 b. 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 slialy limestone of the age of the Lower Helder- 
berg group : Decatur county, Tennessee. 
