5 i6 



NORTH AMERICAN INDEX FOSSILS. 



CX. Modiolodon Ulrich. 



Modioliform, ovate shells, differing from Modiolopsis and Modio- 

 morpha in the possession of one to three oblique cardinal teeth in 

 each valve. Hinge much like that of Ischyrodonta. Ordovicic. 

 419. M. oviformis Ulrich. (Figs. 693 ; 694, a.) Ordovicic. 



Surface marked with a few faint concentric striae. Differs from 



Fig. 694. a, Modiolodon oviformis, hinge of left valve ; b, c, M. patulus, small 

 shell and internal mold of large one ; d, Modiomorpha concentrica, hinge\>f left valve. 

 (Minn. Surv. ) 



Modiolopsis modiolaris in its cardinal teeth, more oval shape, and 

 rounded instead of sinuate basal margin. Type of genus. 

 Basal Trenton of Kentucky and Tennessee. 



420. M. patulus Ulrich. (Fig. 694, b, c.) Ordovicic. 

 Anterior end very short, in the internal mold occupied almost 



entirely by the elevated anterior muscle scar. Wider, more erect, 

 and more uniformly convex than M. oviformis. 



Middle Galena : Minnesota, Iowa. Trenton : Kentucky. 



CXI. Colpomya Ulrich. 

 Inequilateral. Hinge and basal margins subparallel. Broad 

 mesial furrow distinct. Beneath beak of right valve is a tooth- 

 like prominence which fits into a corresponding depression in the 

 opposite valve, differing thus from Modiolodon ; beneath this de- 

 pression is a strong projecting process. Ordovicic. 



421. C. constricta Ulrich. (Fig. 695, a, b.) Ordovicic. 

 Valves very convex on umbonal ridge. Beaks strongly in- 

 curved. Surface marked with strong concentric growth lines. 

 Type of genus. 



Upper Trenton of Kentucky. 



