GASTEROPODS. 195 



At the other extremity are the Capulus-like forms, similar to 

 those described by White and Whitfleld as Platyceras bivolve. 



Strophostylus, as now understood, embraces three rather 

 well-marked types of shells. One of these sections contains 

 chiefly those extreme forms upon which the genus was origin- 

 ally founded. These shells are subglobose, with the spire some- 

 what elevated ; the coluinella parts are prominent, and the 

 front portion of the inner lip is considerably thickened, often 

 having a distinct depression or groove, which continues inward 

 around the columella. This group finds its greatest develop- 

 ment in the Upper Silurian. Another section includes shells 

 similar to Strophostylus ( Platystoma ) niagarensis, in which 

 the spire is depressed, the inner lip simply anchylosed to the 

 body-whorl, and thickened to little or no extent. These forms 

 predominate in the Devonian. They closely approach certain 

 Capuli, which have been called Platycerata, and it is very pro- 

 bable that the generic position of a number of species in the 

 latter genus will be modified upon more critical examinations 

 of all the forms. To the third section belong chiefly Carbon- 

 iferous shells like Strophostylus (Platystoma} peoriensis Me- 

 Chesney. 



Strophostylus reversus (HALL). 



Platyceras reversum Hall, 1860 : Geology Iowa, vol. I, Supp., p. 91. 

 Platyceras reversum Meek & Worthen, 1868 : Geol. Sur. Illinois, vol. II, p. 



508, pi. xv, figs. 4a-b. 



Shell ventricose, subovate ; volutions about two in number, 

 rapidly increasing in size from the apex ; spire very small, de- 

 pressed, somewhat flattened; aperture large, subcircular in 

 outline. Columella narrow, furrowed longitudinally. Surface 

 marked only by lines of growth. 



Horizon and localities. Lower Carboniferous, Keokuk 

 limestone : Boonville ( Cooper county). 



As remarked in the Synopsis of American Carbonic Calyp- 

 tneidie, this form is not a member of the Capulus nor Platyceras 

 group, but manifestly belongs to Strophostylus as now under- 

 stood. Meek's discovery of a well-defined, grooved columellar 

 thickening in the Boonville specimen adds further proof of its 

 affinities to the genus just alluded to. 



