GASTROPODA— PALMACMMIDZE. 



607 



Broadly oval, depressed conical, apex nearly central ; undula- 

 tions strong, subangular. (Type of genus.) 

 Potsdam sandstone of New York. 



16. P. irvingi Whitfield. Cambric. 

 Larger than preceding, but smaller than the next, more nearly 



circular, with compressed apex, and broad but depressed flat 

 undulations. 



St. Croix beds of Wisconsin. 



17. P. quebecensis (Billings). (Fig. 808.) Ordovicic. 





Fig. 808. Palceacmcea qitebecensis, 

 X l /z- (After Billings.) 



FlG. 809. a, b, Palceacmcea humilis ; c, 

 P. (Parmophorel/a) acadica. (a, b, after 

 U. and S., Pal. Minn. ; c, after Walcott, 

 Bull. 10, U. S. G. S. ) 



Large, elliptical, conical, three inches or more in length, with 

 excentric apex, and coarse, shallow concentric undulations. 



Quebec (Beekmantown) of Point Levis, Canada. 

 18. P. humilis U. and S. (Fig. 809, a, b.) Ordovicic. 



Small, broadly subovate, anterior outline semicircular, posterior 

 prolonged. Apex slightly bent forward, a short distance in front 

 of center. Concentric wrinkles few, strong, growth lines faint. 



Stones River and Black River of Minnesota. 



VI. Helcionella * Grabau and Shimer. 



Differs from Palceacmcea in having the apex submarginal and 

 incurved. Concentric folds broader and most pronounced on the 

 side away from the beak ; faint or obsolete under the beak. (Type 

 Metoptoma rugosa Hall.) Cambric. 

 19. H. rugosa (Hall). (Stenotheca rugosa of authors.) (Fig. 



810.) Cambric. 



