146 GASTBROPODS. 



siderably flattened ; suture rather deeply impressed ; aperture 

 subcircular, and somewhat angular in front. 



Horizon and localities. Lower Silurian, Trenton limestone : 

 Glencoe ( St. Louis county ). 



This shell, usually found through the continental interior, 

 occurs as natural internal casts. In both the shell and the cast 

 the secondary revolving angularity, a short distance below the 

 periphery, is frequently not apparent; in other individuals it is 

 barely noticeable, but in the unajority of specimens it is more 

 or less well marked. 



There is scarcely any hesitancy in referring HalFs forms 

 and the specimens described by Shumard as M. carinifera to 

 one and the same species. Unfortunately Hall's name was pre- 

 occupied by McCoy, in 1844, and it becomes necessary to adopt 

 Shumard's term for this shell, though Hall, in 187/, re- 

 christened the species. The specimens used by Shumard in 

 his description were, in all probability, natural casts ; yet the 

 Missouri localities also furnish well-preserved shells, showing all 

 the structural characters. 



Murchisonia gracilis HALL. 



Murchisonia gracilis Hall, 1847: Pal. New York, vol. I, p. 180, pi. xxxix, 

 figs. 4a-c. 



Shell small, very slender, with evenly rounded whorls. 

 Surface smooth, showing only the peripheral band and fine lines 

 of growth. 



Horizon and localities. Silurian, Trenton limestone : Mc- 

 Oune station (Pike county). 



Murchisonia terebra WHITE. 



Plate xlix, fig. 4. 



Murchisonia terebra White, 1833: U. S. GeoL Sur. Terr., 12th Ann. 

 Rep., p. 139, pi. xxxiv, fig. 4. 



Shell very long and slender ; whorls numerous, angulated 

 around the periphery. 



Horizon and localities. Upper Carboniferous, Upper Coal 

 Measures: Kansas City. 



