LEIORHYXCHUS 511 



the series of cross-sections given in figure 11. Externally it is more or 

 less subcnbical in form, with comparatively fine plications and slightly 

 depressed median sinus in the pedicle valve, which is greatly produced 

 anteriorly at nearly a right angle to the plane of the valve. These char- 



Figure 11. — Cross-sections of the rostral Portion of Wilsonia grosvenori (Hall) 



This series of five ci'oss-sections (X ^V^) is from a specimen from the Salem limestone 



of Illinois 



acters give the species much the aspect of members of the genus Wilsonia, 

 to which genus it is here provisionally referred. The internal characters 

 of typical members of the genus Wilsonia differ in no essential respect 

 from Camarotoechia, and if the genus is worthy of recognition it must be 

 distinguished by its external form. 



Leiorhynchus Hall 



leiorhtnchus greeneanum (ulrich) 



Another rhynchonelloid genus recognized by Hall and Clarke, which 

 has the essential internal characters of Camarotcechia, is Leiorhynchus. 

 This genus is established primarily upon its external form, it being but 



Figure 12. — Cross-sections of the rostral Portion of Leiorhynchus greeneanum (Ulrich) 



This series of seven cross-sections (X 2V2) is from a specimen from the Knobstone 

 formation of southern Indiana 



obscurely plicated, with its plications commonly better developed on the 

 fold and sinus than upon the sides of the shell. A good example of this 

 genus is L. greeneanum (Ulrich), from the basal Mississippian of south- 

 ern Indiana, a series of cross-sections of which is shown in figure 12. 



