PELECYPODA—PRIONODESMACEA. 



409 



120. B. exigua (Meek and Hayden). Cretacic. 



Is proportionally higher, shorter and more gibbous than B. 

 saffordi, but of about same size. 



Pierre : Montana, South Dakota. 



XXXVI. Cyrtodonta Billings. 



Equivalve. Shell convex, usually heavy. Beaks prominent, 

 incurved, situated in the anterior third of the shell. Surface 

 marked with concentric lines. Hinge plate strong. Cardinal 

 teeth two to four, strongly curved, situated mostly in front of the 

 beaks. Posterior lateral teeth two or three, strong, elongate, 

 slightly curved and oblique. Two adductor scars present ; the 



FlG. 528. a-c, Cyrtodonta subovata ; d, e, C. billingsi. (Minn. Surv.j 



posterior the larger but less impressed. Pallial line simple. Or- 

 dovicic and Siluric. 



121. C. billingsi Ulrich. (Fig. 528, d, e.) 



Internal umbonal furrow and 

 ridge present. 



Upper Stones River : Illi- 

 nois, Wisconsin, Minnesota. 



Ordovicic. 



122. C. grandis Ulrich. (Fig. 



529.) Ordovicic. 



Large. Surface smooth, 

 marked with fine concentric 

 growth lines. 



Trenton and Cincinnati 

 Groups : Pennsylvania, Kentucky, Tennessee, Wisconsin, Minne 

 sota, Iowa. 



Fig. 529. Cyrtodonta grandis, X h 

 (Minn. Surv. ) 



