126 



NORTH AMERICAN INDEX FOSSILS. 



LXVI. Clinoceras Mascke. 



Differs from the preceding in its slender, gently arcuate form, 

 and broad constriction below the aperture, which is not contracted. 

 Ordovicic-Devonic. 



173. C. m'umiaeforme (Whitfield). (Fig. 1371.) Ordovicic. 



Slender, of circular section, gradually expanding and gently 



Fig. 1 37 1. Clinoceras miimiceforme , 

 fragment showing siphuncle reduced. 

 (After Clarke, Pal. Minn.) 



Fig. 1372. Fotei-ioceras apertujn. 

 (After Clarke, Pal. Minn.) 



arcuate; constriction below the aperture broad and gentle; siphun- 

 cle excentric, distinctly nummuloidal. 



Stones River beds of Minnesota and the Cincinnati region. 



LXVII. PoTERiocERAS McCoy. 



Breviconic orthoceracones and cyrtoceracones, chiefly character- 

 ized by the short and stout form and contracted subtrigonal aper- 

 ture. Ordovicic-Carbonic. 



174. P. apertum Whiteaves. (Fig. 1372.) Ordovicic. 



Short, compressed ; venter somewhat narrower than the dorsum ; 

 body chamber long, gradually contracted. 



Trenton of Minnesota and Canada. 



