1 64 NORTH AMERICAN INDEX FOSSILS. 



laterally and venter more strongly arched; only about three sharp 

 constrictions to the last volution ; suture with eight lateral lobes 

 on each side deeply digitate. 

 Middle Triassic of Nevada. 



Phylloceratida. 



CXVIII. UssuRiA Diener. 



Compressed, involute, with narrow umbilicus,- and narrowly 

 rounded venter ; surface smooth except for fine spiral striae ; 

 sutures ammonitic, lobes and saddles digitate and highly special- 

 ized; external lobe divided by broad, digitate, siphonal saddle, 

 each side deeply trifid with secondary indentations ; principal lateral 

 lobes two or three, wide, deep and strongly digitate ; auxiliaries 

 smaller. Triassic. 



267. U. waageni Hyatt and Smith. (Fig. 1410, ^,/.) Triassic. 



Strongly compressed ; umbilicus very narrow and almost closed ; 

 young passing through Dimorphoceran stage. 



Meekoceras beds (Lower Triassic) of Aspen Ridge, Idaho. 



CXIX. Phylloceras Suess. 



Involute ammonites with umbilicus small or absent, laterally 

 compressed with rounded venter ; surface smooth or with obliquely 

 forward extending lines or folds, which extend without break 

 across the venter ; varices or constrictions may occur ; suture with 

 numerous lobes or saddles, regularly increasing towards the venter; 



b 



a 



Fig. 141 1. Phylloceras mazapilense, a, section d, suture. (After Burkhardt. ) 



saddles ending in rounded, leaf-like pinnules; lobes numerous. 

 Jurassic-Cretacic. 



268. P. apenninicum Canavari. Jurassic. 



Large, widening rather rapidly, with small umbilicus; surface 



with strong folds on inner side dividing into fine and uniform 



striae on the outer portion; suture highly complicated; divisions 



