4 io NORTH AMERICAN INDEX FOSSILS. 



123. C. subovata Ulrich. (Fig. 528, a-c.) Ordovicic. 

 Hinge rather short. Surface nearly smooth, but with age de- 

 veloping strong marginal growth lines. 



Upper Stones River of Kentucky. Black River of Wisconsin. 



124. C. undulostriata Hall. Siluric. 

 Surface marked with strong concentric folds, covered with fine 



undulating concentric striae. 

 Rochester shale of New York. 



125. C. canadensis (Hall). (Megalomus canadensis Hall). (Fig. 



530.) Siluric. 



Shell extremely thick, ovoid, its depth equal to its height. Sur- 

 face concentrically striated. The space within the shell, beneath 



Fig. 530. Cyrtodonta (^Megalomus') canadensis, internal mold. (After Logan. ) 



the beaks and extending over half way to the base of the valves 

 is very much thickened so that an internal mold shows two pro- 

 jections at the hinge line instead of the single one of the beak as 

 usual in Cyrtodonta. 



Guelph : Ohio, Wisconsin and Ontario. 



XXXVII. Megambonia Hall. 



Equivalve or nearly so, gibbous, inequilateral, with shorter 

 anterior end which bears a strong muscular impression. Posterior 

 extremity expanded and compressed, frequently wing-like. Sur- 

 face marked with concentric growth lines and often with fine 

 radiating striae. Teeth numerous, in the anterior portion of the 

 hinge. 



