198 NORTH AMERICAN INDEX FOSSILS. 



Ancyloceratida. 



(A provisional group of phylogerontic genera.) 



CLIIL Crioceras Leveille. 



Loose coiled, spiral, gerontic ammonites, the coil in a single 

 plane, the whorls not in contact; surface ornamented by ribs and 

 two (typically) or three rows of nodes or spines on either side of 

 median Hne of venter, which latter is smooth or costate. Coman- 

 chic-Cretacic. 



344. C. latum Gabb. (Fig. 1458.) Comanchic. 



Fig. 1458. Crioceras lahau.y^^j^. (After Gabb.) 



Principal ribs strong, rounded, and bearing spines in the young, 

 and three rows of blunt nodes in the adult ; between them are about 

 three finer and lower ribs without tubercles or spines. 



Knoxville and Lower Horsetown of California. 



345. C. percostatum Gabb. Comanchic. 



Large, with subquadrate whorls, very nearly in contact ; surface 

 with simple or dichotomous ribs, small and numerous in young, 

 fewer and large on adult, with broad interspaces, crossing the 

 venter. 



Knoxville and Horsetown of California. 



