2l6 



NORTH AMERICAN INDEX FOSSILS. 



377. S. lobatus (Tuomey). Cretacic. 



Large, with small umbilicus, without umbilical shoulder, sides 

 more strongly arched than in preceding; surface smooth or with 

 obscure, fold-like costse; lobes with numerous marginals. 



Ripleyan of New Jersey, Alabama and Mississippi. 



^nr*****' 



Fig. 149 1. Sphenodiscus lenticularis, small specimen, with septum of a larger one, 

 all X %• (After Meek.) 



378. S. lenticularis (Owen). (Fig. 1491.) Cretacic. 



Similar to preceding, but venter more acute ; saddles and lobes 

 much more strongly incised, the .former bifid, trifid and quadrifid. 



Fox Hills of South Dakota, Wyoming and New Mexico. 



CLXVII. Placenticeras Meek. 



Shell similar in form to Engonoceras, except in forms transi- 

 tional to Stantonoceras, where the adult whorl becomes rounded. 

 The venter of the young is flat, then concave and bordered by 

 sharp ridges, and subsequently these become tuberculated, though 

 in old age the venter may become smooth, flat, and even rounded ; 

 surface smooth or tuberculated ; suture complex, of numerous 

 more or less deeply incised lobes and saddles, the third lateral 

 lobe the deepest, after which there is an abrupt decrease. Cretacic. 



