620 



NORTH AMERICAN INDEX FOSSILS. 



57. B. nactus Hall. Devonic. 

 More abruptly rounded and with sharper keel ; aperture less 



abruptly expanded. 



Chemung of New York and Pennsylvania. 



58. B. sublaevis Hall. (Fig. 832.) Mississippic-Carbonic 



Fig. 832. Bellerophon sublczvis. (After Whitfield, Bull. Am. Mus. Nat. Hist.) 



Medium-sized, subglobose, non-umbilicate, with transverse aper- 

 ture and an extended thickened lip, with deep emargination and 

 low rounded dorsal carina. Surface smooth except for growth 

 lines, which, together with the carina, are often obsolete. 



St. Louis and Chester groups of Indiana, Illinois, Ohio, Missouri, 

 and Arkansas, and Lower Coal Measures of Pennsylvania. 

 59. B. crassus Meek and Worthen. (Fig. 833.) 



Carbonic-Permic. 



Large, subglobose, thick-shelled, slightly umbilicated, with nar- 



Fig 833. Bellerophon crassus. (After White, 13th Ind. ) 



row, prominent, subangular keel ; inner lip strong, spreading over 

 umbilicus ; surface with growth lines and fine wrinkles. 



