178 GASTEROPODS. 



Capulus equilateralis (HALL). 



Plate lii, figs. lOa-b. 



Platyceras eguilatera Hall, 1858: Geology Iowa, vol. I, Supp., p. 89. 

 Platyceras eguilatera Meek & Worthen, 1873: Geol. Sur. Illinois, vol. V, 



p. 518, pi. xvii, fig. 2. 



Platyceras equilateralis Miller, 1877: Cat. Am. Pal. FOBS., p. 156. 

 Platyceras eguilatera White, 1880: Geol. Sur. Indiana, p. 514, pi. vii, fig. 5. 

 Platyceras eguilatera Keyes, 1888: Proc. Am. Philosophical Soc. , vol. 



XXV, p. 236, figs. 2 and 3. 



Platyceras eguilatera Keyes, 1889: American Geologist, vol. Ill, p. 331. 

 Platyceras equilatera Keyes, 1889: Proc. Acad. Nat. Sci., Phila., p. 288. 

 Capulus equilateralis Keyes, 1890: American Geologist, vol. VI, p. 9. 

 Capulus equilateralis Keyes, 1890: Proc. Acad. Nat. Sci., Phila., p. 171, 



pi. ii, fig. 11. 



Shell of medium, but often attaining a large size ; extremely 

 variable, hemispherical to oblique conical, with incurved spire ; 

 volutions one to two in number, free or contiguous, moderately 

 enlarging for some distance from the apex, and then rather 

 abruptly and rapidly expanding. Aperture very large, broadly 

 oval, or often nearly circular ; lip thin, more or less undulat- 

 ing. Surface smooth, but toward the apertural margin exhib- 

 iting numerous, often strongly imbricating, sinuous lines of 

 growth. Frequently many small obscure longitudinal folds 

 are also present. 



Horizon and localities. Keokuk limestone and shales : 

 Way land (Clarke county) ; Keokuk and Bonaparte ( Iowa ); War- 

 saw and Niota ( Illinois ). Burlington limestone : Burlington 

 (Iowa); Springfield (Greene county). 



This species seems to be one of the most abundant gas- 

 teropods of the Keokuk beds. Some of the shells often attain 

 very considerable measurements: height, 45 mm.; length along 

 the dorsum, 95 mm.; breadth, 60 mm. Not only is the species 

 under consideration variable in size, but it is extremely so in 

 form and in the configuration of the apertural margin. Per- 

 haps no Capulus in all the Carboniferous presents so wide a 

 range of variation as does this species. Immature shells ap- 

 pear to be glabrate, but as growth proceeded they became more 

 and more rugose and imbricate. The spire is as often contig- 

 uous as free and simply incurved ; and in adult specimens it 



