154 



NORTH AMERICAN INDEX FOSSILS. 



244. M. (Prionolobus) jacksoni Hyatt and Smith. (Fig. 1403, d-f.) 



Triassic. 



Venter rounded ; umbilicus moderately large ; surface smooth ; 

 a long, straight row of denticulations in place of fourth lateral 

 lobe. 



Lower Triassic (Columbites bed) of Idaho and California (?). 



245. M. (Beyrichites) rotelliforme Meek. (Fig. 1403, ^7-^.) 



Triassic. 



With small, deep umbilicus, abrupt almost rectangular umbilical 



shoulder ; whorls widest near umbilicus, ' rapidly sloping to the 



Fig, 1403. a-c, Meekoceras gracilistriatum, two views of a typical specimen and 

 ventral view of another X >2 ; d-f, M. (^Prionolobus) jacksoni, views of two speci- 

 mens X /^» ^"<J^ suture ; g-i, M. {^Beyrichites) rotelliforme, two views X ^> ^.nd 

 suture enlarged ; 7, k, M. [IConinc kites) niushbachanum, lateral view X ^j ^"^ 

 suture ; l-n, M. ( Gyronites) aplanatum, two views, and suture X /^- (After Hyatt 

 and Smith. ) 



sharply rounded venter; surface with numerous close-set ribs, 

 widening to venter; saddles slightly indented. 



Middle Triassic of West Humboldt Range, Nevada. 



