414 



NORTH AMERICAN INDEX FOSSILS. 



133. V. dixonensis Meek and Worthen. (Fig. 535, a-c.) 



Ordovicic. 

 More oblique, more convex, and with more rounded hinge ex- 

 tremities than V. umbonata. 



Upper Stones River of Illinois, Wisconsin, Minnesota. 



134. V. rotundata Hall. (Fig. 535,7.) Ordovicic. 

 More erect, with more circular outline than V. dixonensis. 

 Upper Stones River of Wisconsin, Minnesota ? 



135. V. umbonata Ulrich. (Fig. 535, d-f.) Ordovicic. 

 Shell substance rather thin. Cardinal teeth long. 



Black River : Kentucky, Minnesota. 



136. V. hayniana Safford. (Fig. 535, k.) Ordovicic. 

 Not so high as V. umbonata and with longer hinge line. 

 Trenton : Pennsylvania, Kentucky, Tennessee, Minnesota. 



XL. Whitella Ulrich. 

 Differs from Cyrtodonta in its thinner shell, the presence of a 

 longitudinally striated area extending posteriorly from the beaks, an 



FlG. 536. a, b, Whitella scqfieldi, left valve and its hinge ; c, d, W. quadran- 

 gularis ; e,f, W. ventricosa, right and anterior views of internal mold. (Minn. Surv. ) 



