CEPHALOPODA— AMMONOIDEA. 



227 



umbilical tubercle. The ribs and nodes become obsolete towards 

 the venter in the old shells. 

 Ripleyan of New Jersey. 



rw^ 



Fig. 1508. Mortoniceras ddlawarense, lateral view of adult, and septum, X y^- 

 (After Whitfield, Pal. N. J., II.) 



CLXXII. Prionotropis Meek. 



Young similar to Mortoniceras, but ribs with tubercles only at 

 their outer ends; in adult, the keel often becomes broken up into 

 elongate, discontinuous node-like sections and the nodes become 

 strong, blunt spines; sutures relatively simple, with few, broad, 

 bifid saddles and lobes of similar width. Cretacic. 



400. P. woolgari (Mantell). (Fig. 1509, a-J.) Cretacic. 



Scarcely increasing in width, tubercles of adult coarse ; keel 

 broken up into corresponding sections; suture with deeply bifid 

 first lateral, and obtuse siphonal saddles. 



Benton of South Dakota, Missouri River region, Nebraska, 

 Colorado, New Mexico; Eagle Ford shales of Texas; Turonian 

 of Europe. 



