354 



NORTH AMERICAN INDEX FOSSILS. 



415. C. roissyi (L'Eveille). Mississippic. 

 Differs from C. hirsuta in the unequal convexity of the valves, 



the brachial valve being regularly convex while the pedicle valve 

 is convex above (near beak), but below is depressed and often 

 flattened. It differs also in the scarcely incurved beak of the pedicle 

 valve and in the character of the surface. 



Keokuk-Kaskaskia of Mississippi Valley and western United 

 States. 



416. C. hirsuta Hall. (Fig. 464.) Mississippic. 

 Nearly equally biconvex. Beak of pedicle valve prominent and 



incurving over that of brachial valve. Surface marked by concen- 

 tric imbricating lamellae on which rise rows of minute spines. 



Fig. 464. Cliothyris hirstda (Ind. Geol. Survey). 



Sinus and fold absent, being represented merely by a slight de- 

 pression in the front of the pedicle valve. 



St. Louis and Kaskaskia of Kentucky, Indiana, Illinois, Montana. 



CXXIII. Seminula McCoy (emend. Hall and Clarke). 

 Small. Differs from AtJiyris in the presence of a median sinus 

 oon the pedicle valve and a fold on the brachial with often an 



obscure fold on each side 

 on both valves. Surface of 

 valves smooth, never lamel- 

 lose. Mississippic-Carbonic. 



417. S. subquadrata Hall. 

 Mississippic. 

 Subquadrate with length 

 and breadth nearly equal. 

 Brachial valve slightly tljie 

 more convex and bearing a broad, mesial fold. Sinus of pedicle 

 valve broad and deep and produced anteriorly. Surface marked 

 concentrically with fine growth lines and toward the margin by 

 strong lamellae. 



Kaskaskia of Ohio, Kentucky, Illinois, Utah. 



FlG. 465. Seminula trinudeus (Ind. Geol. 

 Survey). 



