6i8 



NORTH AMERICAN INDEX FOSSILS. 



foundly carinated ; narrower and less rapidly enlarging than pre- 

 ceding species ; surface striae strongly zigzag, rather distant, and 

 subimbricating, with finer striae between. 



Trenton of New York and adjoining Canadian regions. 

 51. P. dyeri Hall. (Fig. 828, e t f.) Ordovicic. 



Small, whorls embracing about one third, rounded laterally and 

 less strongly carinated than in preceding. Surface with spirals due 

 to regular emargination of lamellae. 



Richmond group of Indiana, Ohio, Kentucky, and Minnesota. 



XIX. Bellerophon Montfort. (Emend. Waagen.) 

 Subglobose, umbilicus mostly small or absent, dorsum rounded, 

 aperture generally expanded, usually with a callosity on the inner 

 lip. A central emargination and slit band or elevated blunt 

 (rarely noded) keel present ; surface marked only by strong 

 growth lines, rarely by one or more rows of nodes. Ordovicic- 

 Permic. 

 52. B. troosti (d'Orbigny) SafTord. (Fig. 829, a-c.) Ordovicic. 



FlG. 829. a-c, Bellerophon troosti; d-g t B. platystoma (g, cross-section of 

 whorl). (After Ulrich, Pal. Minn.) 



Whorls rounded, rapidly enlarging, flaring at lip ; umbilicus 

 very small, sharp and deep ; dorsum with distinct rounded keel in 

 adult. 



Trenton of Tennessee and Kentucky. 



