Description. — Broad-oval, the rostral extremity obtusely pointed, front 

 broadly rounded, greatest -^vidtli a little below the middle ; both valves 

 rather strongly and uniformly convex, most tumid at about one third the 

 length from the beak. Ventral valve more acute above than the dorsal, 

 beak depressed below the greatest elevation of the shell, slightly elevated 

 above the margin, with a small area beneath it which is inclined backward 

 at an angle which varies from 45° to 60°. Dorsal valve with an obtusely 

 rounded iimbo, the beak scarcely distinct from the cardinal edge and not 

 elevated above the margin. Surface vrith fine concentric strise or small 

 minutely rugose ridges of growth of variable size, from 4 to 8 in one line, 

 often smooth from exfoliation, or Avearing. Colour of tlie shell in the red- 

 dish limestone a honey yellow, in grey hmestone greyish ; when exposed 

 to the weather becomes white and minutely fibrous. 



Length and breadth about three lines. 



In some specimens the ventral valve is depressed convex, the beak 

 being on a level Avith the greatest elevation of the shell. The shell is 

 thick and strong, and when well preserved, breaks with a granular fracture. 

 When weathered, a tendency to fibroiis exfohation is manifested. 



This species is closely allied to the form that is found so abundantly in 

 the Troy limestone, but the muscular impressions in that one are rather 

 closer together and nearer the beak. (At least they are so in the speci- 

 mens in my possession.) 



Many of the specimens are a little more obtuse in the upper half than 

 those figured. The individuals are exceedingly numerous and difier little 

 in size. 



Lociilitji and Forinalion. — Anse an Loup, on the north shore of the 

 Straits of Belle Isle. In limestone of the Potsdam group. 



Collector. — J. Richardson. 



ObOLELLA rKuTORGINA) CIXGTTLATA. (N. Sp.) 



;'?^^i;f>^. 



Pig. 8. 



Fig. 9. 



Pig. 8. — Ventral (?) valve of O. cingulata. Pig 

 Fig. 10.— Dorsal (?) valve. 



Pig. 10. 

 -Cast of interior of ventral valve. 



