334 



NORTH AMERICAN INDEX FOSSILS. 



very prominent and strongly incurved beak. Plicatious many, 

 covering the entire shell and marked by fine radiating and con- 

 centric striae. 



Keokuk of Ohio, Illinois, Iowa, Utah. 



364. S. centronatus A. Winchell. (Fig. 425, c.) Mississippi. 



Of medium size, broadest at hinge line, often mucronate. 

 Valves moderately convex. Beak of pedicle valve strongly 

 elevated and incurved. Surface with 34 to 42 small plications of 

 which 4 to 6 mark sinus and fold. 



Waverly of Ohio, Michigan, South Dakota, Utah, Wyoming, 

 Nevada. 



Fig. 425. a-b, Spiriferkeokuk. 

 e-f Spirifer increbescens. 



, Spirifer centronatus. d, Spirifer marionensis. 

 (After Hall, Nettelroth and Pal. Ohio. ) 



365. S. marionensis Shumard. (Fig. 425, d.) Mississippi. 



Nearly semicircular with hinge line extended in mucronate 

 points. Valves nearly equally convex. Fold and sinus marked 

 by 2 or 3 dichotomizing plications. Cardinal areas narrow. Sur- 

 face covered with many plications which irregularly bifurcate. 

 Granulose. 



Chouteau of Ohio, Missouri. 



