502 Systematic Paleontology 



Discohelix has been reported from strata as old as the Trias and from 

 strata as young as the Oligocene. The type species was described from 

 the Lias. 



Discohelix lapidosa (Morton) 



Delphinula lapidosa Morton, 1834, Synop. Org. Rem. Cret. Group U. S., p. 46, 



pi. xxx, fig. 7. 

 Delphinula lapidosa Whitfield, 1892, Mon. U. S. Geol. Survey, vol. xviii, p. 



152, pi. xvii, figs. 9-11. 

 Straparolus lapidosus Johnson, 1905, Proc. Acad. Nat. Sci., Phila., p. 19. 



Description. — " Shell discoidal, with about three volutions ; shoulder 

 angulated; margin flattened; umbilicus profoundly patulous; spire 

 slightly elevated above the body whorl." — -Morton, 1834. 



Type Locality. — Prairie Bluff, Alabama. 



This shell is unusually well characterized by its rapidly enlarging 

 depressed whorls and flattened apical surface. It is quite widely dis- 

 tributed through the Upper Cretaceous of eastern North America, but is 

 nowhere abundant. 



Occurrence. — Monmouth Formation. Brooks estate near Seat Pleas- 

 ant, Prince George's County. 



Collection. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, U. S. National Museum. 



Outside Distribution. — Ripley Formation. Exogyra costata zone, 

 Union County, Mississippi. Selma Formation. Exogyra costata zone, 

 Kemper and Oktibbeha counties, Mississippi. 



Genus AMAUROPSIS Morch 

 [Moll. Gronl. Nat. Bidr. Beck's Gronl., 1857, p. 81] 



Type. — Nerita islandica Gmelin. 



Shell rather small, thin, in the recent shells covered with a conspicuous 

 periostracum ; outline ovate; spire elevated for the group; external sur- 

 face smooth or feebly sculptured; sutures channelled; aperture holosto- 

 mous, obovate, inner lip oblique or feebly excavated ; parietal wall usually 

 glazed ; umbilicus nearly or quite imperforate ; operculum horny. 



Etymology: Amaura, a pyramidellid genus; oi/-is, form. 



