238 NORTH AMERICAN INDEX FOSSILS. 



circular. Length less than one-half breadth. Hinge line slightly- 

 longer than greatest width of shell below. Pedicle valve with a 

 broad and shallow mesial sinus which is at times wanting. Cardinal 

 margin with four to seven oblique spines on each side of the 

 beak. Surface smooth except for faint concentric striae. 

 Kansas, Iowa, Illinois, Colorado, Nebraska. 



131. C. granulifer Owen. Upper Carbonic. 

 Rather large (about ^ inch wide). Distinguished by the broad 



shallow sinus in the pedicle valve, the often extended hinge line 

 and the presence of six to ten oblique spines on each side of the 

 beak. Surface marked by very fine radiating striae. 



Alabama, Missouri, Illinois, Iowa, Kansas, Colorado, Arizona. 



132. C. mesolobus Norwood and Pratten. (Fig. 285, i.) 



Upper Carbonic. 

 Small. Distinguished by the presence of a lobe or fold in the 

 mesial sinus of the pedicle valve. 



Ohio, Missouri, Illinois, Colorado, New Mexico, Arizona. 



133. C. variolatus (d'Orbigny). Upper Carbonic. 

 Similar to C. granulifer but more convex, very small and with 



the sinus of the pedicle valve well-marked. 



Ohio, Illinois, Missouri, Kansas, Nebraska, Colorado. 



134. C. verneuilianus Norwood and Pratten. (Fig. 290.) 



Upper Carbonic. 



Small and much wider than long with extended and sometimes 



mucronate hinge extremities. Pedicle valve bearing a deep, 



rounded mesial sinus, giving the valve a 

 two-lobed appearance. Hinge line with 

 four oblique spines on each side of the 

 beak. Brachial valve bearing an obtuse 

 median fold. Surface marked by many 

 fine radial striae. Especially distinguished 

 Fig. 290. Chonets ver- from other species by its deep sinus and 

 netalianus, two pedicle , ., , , 



valves showing variation, bilobed appearance. 



(Ind. Geol. Surv. ) Indiana, Illinois, Missouri, Kansas, Ne- 



braska, Colorado. 



XLIII. Chonostrophia Hall and Clarke. 

 Differs from Chonetes in having the pedicle valve concave and 

 the brachial convex ; in the absence of a median septum in the 

 brachial valve and in the bilobed cardinal process. 



