SPIXtIFERiE OF THE HAMILTON GROUP. 
235 
Dorsal valve less gibbous than the ventral, somewhat regularly convex 
in the middle, and curving towards the front and baso-lateral margins; 
a little depressed or flattened towards the abruptly rounded cardinal 
extremities : umbo slightly elevated above the hinge-line ; area linear. 
There is often a faint impressed line extending from the beak to the 
base of the valve. 
Surface marked by fine concentric lines of growth, which are sometimes 
crowded into imbricating folds towards the front of the shell. In par¬ 
tially exfoliated specimens the surface is finely and distinctly punc¬ 
tated, as if in its original condition it had been covered by closely 
arranged spinules. The texture of the shell is fibrous, but yet differing 
from the fibrous texture of ordinary Spirifers. 
This species is a miniature of the S. lineata of the Upper Carboniferous beds, 
having the same general aspect and characters of sui’face marking : it is, however, 
usually a little more extended laterally, and the area continues more nearly to the 
cardinal extremities.The punctate markings are comparatively finer, and it has 
not the regular concentric undulations which often mark the carboniferous species. 
It has likewise a distinct impressed line or narrow sinus in the dorsal valve. 
The ventral valve alone has much the appearance of Amboccelia, and I had thus 
referred the species from some crushed and imperfect specimens. Farther exami¬ 
nations have brought out specimens in their true form and proportions; not only 
from the calcareous beds near the base of the Hamilton group, but also from the 
Tully limestone we have specimens undistinguishable from this one. 
Geological formation and localities. In the calcareous bands of the Hamilton 
shales, on the shore of Lake Erie; in the Tully limestone near Tully, and from 
the same rock in Seneca county. 
Sjpirifera arata. 
Svirifer arata : Hail, Tenth Report on the State Cabinet, p. 161. 1857. 
I have already referred to this species under S. granulifera. The original speci¬ 
mens designated as S. arata , upon re-examination, do not offer satisfactory means 
for separation from S. grgnulifera ; and therefore it becomes unnecessary to con¬ 
tinue the name. 
