CEPHALOPODA— AMMONOIDEA. 



187 



318. P. colfaxi Gabb. Jurassic. 

 Discoidal, scarcely embracing, increasing very slowly in diam- 

 eter; sides somewhat compressed; in neanic stage every second 

 alternate rib bifurcates, generally at about the middle of the side, 

 but it may vary from near umbilicus to ventral border ; sometimes 

 several single costse occur together. 



Upper Jurassic (Goldbelt slates) of California. 



319. P. skidegatensisWhiteaves. (Figs. 1444, 1445.) Comanchic. 



Fig. 1444. Perisphinctes skidegatensis, X /^- (After Whiteaves, Mes. Foss., I.) 



Similar to the preceding, but with the single costae not extending 

 to the umbilicus. 



Queen Charlotte formation of Queen Charlotte Islands, Canada. 



CXLL Idoceras Burckhardt. 



Differs from Perisphinctes in having the costae bending forward 

 along the median ventral line, where they become faint or obsolete, 

 a smooth band remaining ; suture very simple, characterized by the 

 predominance of the first lateral lobe. Jurassic. 



