. CEPHALOPODA— AMMONOIDEA. 183 



Arietida. 



CXXXIV. CoRONiCERAS Hyatt. 



Flat, discoidal ammonites, with quadrangular whorls, slightly or 

 scarcely at all embracing ; venter flat with median keel bounded by 

 two pronounced depressions; sides marked by strong ribs which 

 bend forward at the venter and end abruptly at the margin of the 

 ventral grooves, or end in tubercles before reaching the groove; 

 suture with broad saddles moderately incised ; siphonal saddle long 

 and narrowing with slight incisions; second lateral saddle larger 

 than first ; antisiphonal lobe long and ending in two points. Young 

 whorls stout and smooth, followed by tuberculated stage, in form 

 subquadrangular. Jurassic (Lias). 



306. C. claytoni Hyatt. Jurassic. 

 Ribs well defined, lyre-like, with similar tubercles on the angu^ 



lations ; channels deep, somewhat narrow and smooth ; keel promi- 

 nent. 



Lower Lias of Nevada. 



CXXXV. Arnioceras Hyatt. 



Very similar to preceding, differing mainly in the young. Dis- 

 coidal, with quadragonal whorl; costae prominent, thin, sharp, 

 straight and smooth ; geniculations abrupt, on the level with the 

 venter ; first three or four whorls smooth, followed by lateral folds, 

 which later develop into costse, and a median angular ridge, which 

 later develops into a keel; section of young whorl ovoid, higher 

 than wide. Jurassic. 



307. A. nevadanum (Gabb). Jurassic. 

 Large, but slightly embracing; whorls slowly and regularly en- 

 larging; costse round, somewhat more distinctly separated than 

 their width; curving slightly near venter, where they are united 

 by a continuous ridge ; ventral grooves wider than keel ; section 

 quadrate, the whorls higher than wide ; superior lateral lobe 

 bifurcate. 



Lower Lias of Nevada. 



308. A. humboldti Hyatt. Jurassic. 

 Costae higher than in preceding; whorls much higher than wide. 

 Lias of Humboldt County, Nevada, and Inyo County, California. 



