1 86 NORTH AMERICAN INDEX FOSSILS. 



pair of first dorsal saddles, usually two additional pairs of saddles, 

 and two pairs of lobes. Jurassic-Cretacic. 



313. P. mclachlani Burckhardt. Jurassic. 

 Ribs bifurcate, rarely one remains single ; whorls slightly wider 



than high. 



Kimeridgean of Mazapil, Mexico. 



314. P. nikitini Michalski. Jurassic. 

 Transverse section of last whorls nearly circular ; ribs similar to 



preceding, rather coarser and more irregular. 



Portlandian of Mexico; Lower Volgien of Central Russia. 



315. P. mazapilensis Castillo and Aguilera. Jurassic. 

 Higher than wide; sides flattened; costse bifurcating and occa- 

 sionally tri furcating, when the branches on opposite sides do not 

 regularly correspond. 



Upper Jurassic of San Luis Potosi, Zacatecas and Durango, 

 Mexico. 



316. P. felixi Castillo and Aguilera. (Fig. 1443.) Jurassic. 



\%ih. 



Fig. 1443. Ferisphinctes fe/ixi, hagmtni, y^y^,. 



Height and breadth of whorls nearly equal, narrowing towards 

 venter; involution from one half to one third; costse heavy, bifur- 

 cating, more rarely tri furcating at about the middle, sometimes 

 uniting at the umbilical margin, curving forward and making an 

 obtuse angle at the median Hne. 



Malone formation of Texas, Tithonian of San Luis Potosi, 

 Mexico. 

 317. P. virgulatiformis Hyatt. Jurassic. 



Strongly involute, embracing about one third; inner whorls dis- 

 coidal, outer whorls broaden rapidly ; bifurcation of costse irregular. 



Mariposa formation of California. 



