CEPHALOPODA— AMMONOIDEA. 191 



Fig. 1449. Olcostephanus traski, two thirds natural size. (After Gabb. )^ 



widely separated by concave spaces, and the shorter ones do not all 

 unite with them. 



Portlandian of Canyon de San Matias, Santa Rosa, Mexico. 



CXLV. AULACOSTEPHANUS Sutncr and Pomp. 



Similar to preceding, but division of costae takes place at ventro- 

 lateral angles and they cross the venter only in the coronate young, 

 when section is trapezoidal; venter of adult with smooth, median 

 band; suture similar to the preceding; differs from Idoceras in 

 characters of the young shell and in the suture. Jurassic. 



330. A. zacatecanum Burckhardt. Jurassic. 



Venter flatly rounded with broad, smooth space; ribs coarse, 

 especially at the umbilicus becoming broad and distinct before tri- 

 furcating or quadrifurcating near ventral margin; in last part of 

 last whorl they become obsolete ; umbilicus large, deep. 



Kimeridgian of Santa Rosa, Mexico. 



CXLVI. HoPLiTES Neumayr. 



Discoidal, involute shells ; costae bifurcating on sides at umbilical 

 shoulders, generally with tubercles at bifurcation and ends of 



