SPIRIFER.^ 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. Uneata of the Upper Carboniferous beds, 

 having the same general aspect and characters of surface marking : it is, however, 

 usually a little more extended laterally, and the area continues more nearly to the 

 cardinal extremities. The punctate markings are comparatirely 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 iu 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 

 Tally 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. 



Spirifera arala. 



Spiri/er arata : Uxtt, 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 ofier satisfactory means 

 for separation from 8. granulifera ; and therefore it becomes unnecessaiy to con- 

 tinue the name. 



