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NORTH AMERICAN INDEX FOSSILS. 



CXXXVI. Oppelia Waagen. 



Discoidal and highly involute ammonites, with living chamber 

 rounded on venter, the earlier parts acute; sides with weak ribs; 

 lobes asymmetrically divided. Jurassic-Comanchic. 

 309. 0.(?) fallax Castillo and Aguilera. (Fig. 1440.) Jurassic. 



Compressed, with small but deep umbilicus; greatest thickening 

 of shell at umbilicus ; surface smooth. 



Malone formation of Texas, also Mexico. 



CXXXVII. OEcoTRAusTES Waagen. 



Differs from the preceding in the distinct geniculation of the 

 ribs, with a median depressed line connecting the angles. Jurassic. 



Fig. 1440. , Oppelia {?) fallax, 

 young shell, X/^- (After Cragin. ) 



Fig. 1 44 1. Macrocephalites epigo- 

 nus, suture. (After Burckhardt. ) 



310. 0. denticulata Hyatt. Jurassic. 



Smooth and denticulated; suture with short abdominal lobe, 

 large siphonal saddle, large first lateral lobes, with three long 

 slender terminal lobes. 



Mariposa slates of CaHfornia. 



Dactylioida. 



CXXXVIII. Macrocephalites Sutner. 



Strongly involute shells with rounded sides and venter, small 

 umbilicus, and surface ornamented by numerous slightly flexuous, 

 bifurcating ribs which pass across the venter; sutures very com- 

 plex, with lobes and saddles about equal in breadth. Jurassic. 



311. M. epigonus Burckhardt. (Fig. 1441.) Jurassic. 



Height and width of whorls nearly equal; strongly involute, with 

 narrow, deep umbilicus in the younger whorls; ribs rather coarse, 

 but becoming obsolete towards the umbilicus. 



Kimeridgean of Sierra Santa Rosa, Mexico. 



