4i6 



NORTH AMERICAN INDEX FOSSILS. 



XLI. Plethocardia Ulrich. 

 Very similar to Whitella but differs from that genus in the large 

 and strong cardinal process which projects forward and downward 

 from just beneath the beak in each valve ; also differs in the deeply- 

 impressed anterior adductor scar mar- 

 gined on the inner side by a curved 

 ridge extending from the under side of 

 the cardinal process. Ordovicic. 



142. P. umbonata Ulrich. (Fig. 537, 

 d-f.) Ordovicic. 



Type of the genus. 

 Black River : New York, Ken- 

 tucky, Minnesota. 



XLII. Ischyrodonta Ulrich. 

 Valves thick. Beaks small, ante- 

 rior. Hinge plate wide, without pos- 

 terior lateral teeth but with two strong 

 cardinal teeth in the left valve and one 

 large one in the right. Ligament in- 

 ternal posterior to the beaks. Anterior 



exterior of right valve. (Geol. musde scar sharply defined on the in- 



ner side by a ridge extending from 



the cardinal teeth to the base of the scar. Ordovicic. 



FiG. 538. Ischyrodonta union 

 oides, interior and internal mold 



143. I. unionoides Meek. (Fig. 538.) 

 Surface marked with a few distant growth lines. 

 Middle beds of Cincinnati Group : Ohio, Kentucky. 



144. I. modioliformis Ulrich. (Fig. 539.) 

 Elongate. Surface marked with sub- 

 regular concentric furrows and ridges. 



Upper beds of Cincinnati Group: Indi- 

 ana. 



Ordovicic. 



Ordovicic. 



145. I. truncata Ulrich. (Fig. 540.) 



Ordovicic, 



Subquadrate. Posterior margin trun- 

 cate rounded. . Surface marked with a 

 few sublamellose growth lines. 



Near top of Cincinnati Group: Ohio, Indiana 



Fig. 539. Ischyrodonta 

 modioliformis. (After Ulrich, 

 Geol. Ohio, VII.) 



