226 



NORTH AMERICAN INDEX FOSSILS. 



1 



of tubercles in addition to the marginal row of larger ones, and a 

 fifth row of compressed tubercles bounding the ventral grooves; 



Fig. 1506. Mortoniceras texamitn. (After Roemer, 



keel strong, narrow ; in the adult the ribs become broader and the 

 nodes fainter. 



Austin limestone of Texas. 



Fig. 1507' Mortoniceras delawarense, veniral and lateral views of an 

 immature specimen. 



399. M. delawarense Morton. (Figs. 1507, 1508.) Cretacic. 



Ribs more numerous with intercalated secondary ones, which do 



not reach the umbilicus, or with two ribs starting from a single 



