1890.] NATURAL SCIENCES OF PHILADELrHIA. 171 



distance from the apertural margin nearly straight. S[)ire laterally 

 more or less compressed ; sometimes small and short, sometimes long, 

 attenuate, simply incurved or enrolled. Aperture oval, or sub-cir- 

 cular ; margin t<harp, sinuous. Surface marked by somewhat im- 

 bricating lines of growth, and several obscurely defined longitudinal 

 plications, the anterior one being usually larger than the others, and 

 often forming a ])rominent subangular ridge. 



Horizon and localities. Keokuk limestones and shales : Warsaw 

 and Nauvoo, Illinois. St. Louis limestone : Spurgeon Hill and 

 Bloomington, Indiana ; Tuscumbia, Alabama. 



This form appears to have a geographically wide distribution ; 

 and it also presents considerable variation, even within limited areas. 

 It was originally described from Spurgeon Hill, Indiana, and like all 

 the founal remains of that locality is characteristically depauperate. 

 PlaUjceras uncum M. & AV. seems to be identical with this species, 

 the imposed conditions of environment being more favorable to a 

 normal development ; and to the attainment of somewhat larger pro- 

 portions. 

 Capulus equilateralis (Hall). [Plate II, fig. 11.] 



Platyceras equilatera Hall, 1859. Geol. Iowa, vol. I, pt. ii, Supp., p. 89. 



Platyce7-as equilatera Meek & Worlhen, 1873. Geol. Sur. Illinois., vol. V, p. 

 518, pi. xvii, fig. 2. 



Platyceras equilateralis Miller, 1877. Cat. Am. Palae. Foss., p. 15(3. 



Platyceras equilatera White, 18S0. Indiana Geol. Rept. for 1880, p. 514, pi. 

 vii, fig. 5. 



Platyceras equilateruin Keyes, 1888. Proc. Am. Philosophical Soc, vol. xxv, 

 p. 236, figs. 2 and 3. 



Platyceras equilateruin Keyes, 1889. Am. Geologist, vol. Ill, p. 331. 



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



Capulus equilateralis ¥ieyts, 1890. Am. Geol., vol. V. 



Shell 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 ex- 

 panding. Aperture very large, broadly oval, or often nearly cir- 

 cular ; lip thin, more or less undulating. Surface smooth, but to- 

 ward the apertural margin exhibiting numerous, often strongly 

 imbricating, sinuous lines of growth. Frequently many small 

 obscure longitudinal folds are also present. 



Horizon and localities. Keokuk limestone and shales : Keokuk 

 and Bonaparte, Iowa ; Wai'saw and Niota, Illinois ; Crawfordsville, 

 Indiana. Burlington limestone : Burlington Iowa. 



