PELECYPODA—PRIONODESMACEA. 



415 



umbonal ridge, anterior teeth merely two to five oblique folds and 

 posterior lateral wanting ; internal ligament supported by a ridge 

 in each valve which is at times mistaken for a posterior tooth. 

 Ordovicic-Siluric. 



137. W. megambona Whitfield. (Fig. 537, a, b.) Ordovicic. 

 Hinge line very short. Striated area very short and narrow. 

 Upper Stones River of Wisconsin, Minnesota. 



138. W. scofieldi Ulrich. (Fig. 536, <z, b.) Ordovicic. 

 Outline different and hinge and striated area longer than in W. 



megambona. 



Upper Stones River of Minnesota. 



[ FlG. 537. a, b, Whitella megambona, right valve ; c, W. obliquala, left internal 

 mold; d-f, Plethocardia umbonata. (Minn. Surv. ) 



139. W. ventricosa Hall. (Fig. 536, e,f.) Ordovicic. 

 Very convex. 



Black River-»Trenton : New York, Minnesota ?, Canada. 



140. W. obliquata Ulrich. (Fig. 537, c.) Ordovicic. 

 Subquadrangular, very oblique. 



Cincinnati Group : Ohio, Indiana, Minnesota. 



141. W. quadrangularis Whitfield. (Fig. 536, c, d.) Ordovicic. 

 More convex, less oblique, than W. obliquata with larger beaks 



and wider cardinal area. 



Upper beds of Cincinnati Group : Ohio, Illinois, Minnesota. 



