1890.] NATURAL SCIENCES OF PHILADELPHIA. 173 



Capuli is thus capable of being traced and especially in those examples 

 in which the nodosity of the dome plates of the crinoid have reached 

 a high development. This phenomenon of nonparallelisra of the 

 lines of growth is not therefore indicative of a change in station of 

 the gasteropod as has been suggested at various times. 



It has been clearly shown elsewhere' that shells of C. equllateralis, 

 when adhering to flat surfiices are always very much depressed and 

 have the aperture proportionately much more expanded than the 

 average specimen, while the spire is closely incurved, even touching 

 the body of the shell. When the gasteropod is found attached to 

 strongly convex surfaces, or to the calyces of Platycrinus the shell en- 

 larges less rapidly ; and there is also a tendency in the apex to be- 

 come free from the body-whorl and even to completely uncoil, often to 

 such an extent as to approach closely some forms of the C. Injundl- 

 bultnn type. 



Capulus fissurella (Hall). 



Platyceras fissurella Hall, 1859. Geol. Iowa, vol. I,pt. ii, Supp., p. 90. 



Platyceras fissurella Meek & Worthen, 18T3. Geol. Sur. Illinois, vol. V, p. 

 519, pi. XVII, fig. 4. 



Platyceras fissurella Keyes, 1889. Pioc. Acad. Nat. Sci., Phila., 1889, p. 289. 



Capulus fissurella Keyes, 1890. Am. Geologist, vol. V. 



Shell massive, obliquely depressed conical, enlarging rapidly to 

 the aperture ; from apex to margin straight or somewhat arcuate 

 anteriorly, slightly concave posteriorly. Apex obtuse, situated 

 nearly over the posterior margin of the aperture, which is ample, 

 oval or nearly circular ; lip thick, rounded, very slightly sinuous. 

 Surface marked by numerous gently undulating, somewhat irregular 

 and often subimbricating lines of growth. Occasionally there are 

 obscure indications of small longitudinal folds. 



Horizon and localities. Keokuk limestone : Warsaw and N^au- 

 voo, Illinois. Burlington limestone : Burlington, Iowa. 



The specific name of this form is very inappropriate, having orig- 

 inated in a misconception on pai't of the author of the species as to 

 the true nature of the apical perforation in the type specimen. It 

 has been clearly shown by Meek & Worthen that the aperture in the 

 apex is not a natural opening, but an accidental fracture in the shell. 



Recently typical examples of C. fissurella have been found in the 

 upper Burlington limestone, thus adding another case in support of 

 the view lately" advanced that the faunas of the Keokuk and Bur- 



1 Proc. Am. Philosophical Soc, vol. XXV, p. 236. 



» Keyes: American Journ. Sci., vol. XXXVIII, pp. 189-193. 



