CEPHALOPODA— AMMONOIDEA. 225 



395. S. oregonensis Anderson. Cretacic. 

 Differs from vS". propinqua in its more numerous ribs which 



generally arise in pairs at the umbilicus, in the faint character of 

 the umbilical row of tubercles, more flattened sides, less conspic- 

 uous keel, and more angular abdominal area. 

 Lower Chico of Oregon; abundant. 



396. S. chicoensis Trask. Cretacic. 

 Shell with about 24 simple, distinct ribs, with double row of 



tubercles near outer margin of coil ; ribs do not bifurcate but con- 

 sist of two kinds, the smaller not extending to umbilicus; section 

 of whorl oval. 



Upper Chico of California. 



397. S. gabbi Anderson. (Fig. 1505.) Cretacic. 



Fig. 1505. Schloenbachia gabbi Anderson, with cross section and suture, 

 X %. (After Gabb.) 



Ribs 40 or 50, broad, simple, with three or four rows of rounded 

 tubercles ; venter almost squarely truncate, or slightly depressed, 

 broad; keel very low, sides nearly flat. 



Upper Chico of California. 



CLXXI. MoRTONiCERAS Grossouvre. 



Similar to Schloenbachia but with the keel strong and bounded 

 by grooves, and the simple, curving ribs broken up into numerous 

 nodules. In some adult or old-age individuals, the tubercles and . 

 costae become obsolete, before the venter is reached. Cretacic. 



398. M. texanum (Roemer). (Fig. 1506.) Cretacic. 



Type of the genus ; whorls scarcely impressed, with three rows 

 ^5 



