GASTEROPODS. 183 



became relatively smaller, leaving a sharp sub-angular ridge 

 around the shell, parallel to the apertural margin. This ab- 

 rupt decrease in the expansion of the shell imparts to the nat- 

 ural internal casts the appearance of an apical truncation or 

 fracture. 



In its attachment to Paleozoic crinoids, the only form 

 with which I. quincyense has thus far been found associated 

 is Physetocrinus ventricosus ( Hall) a species having a rather de- 

 pressed hemispherical dome, in which the ventral opening has 

 a subcentral location. The dome-plates are small and numer- 

 ous, and frequently studded with small prominent tubercles or 

 sub-spinous processes, which impart to the gasteropod shell 

 series of minute corrugations extending over each of the 

 larger folds. 



Igoceras capulus HALL. 



Plate lii, figs. la-b. 



Platyceras capulus Hall. 1859 : Geology Iowa, vol. I, Supp., p. 19. 

 Meloptoma umbella Meek & Worthen, 1866: Proc. Acad. Nat. Sci., Phila., 



p. 267. 

 Metoptoma umbella Meek & Worthen, 1868: Geol. Sur. Illinois, vol. Ill, 



p. 506, pi. xv, figs. 6a-c, and 7. 



Shell conical, much depressed, with a few broad, more or 

 less obscurely denned longitudinal folds ; apex nearly central. 

 Aperture very large, circular in outline ; lip somewhat undulat- 

 ing. Surface marked only by lines of growth. The muscular 

 scars, as observed in some specimens, consist of curved, oblong 

 impressions on each side, united behind by a narrow band. 



Horizon and localities. Lower Carboniferous, Burlington 

 limestone : Hannibal, Bitter station ( Greene county). 



Although much more depressed than any other known 

 form of the genus, there seems to be but little doubt, at pres- 

 ent, that the shell under consideration properly belongs to the 

 same group as /. pabulocrinus of Owen. Hairs original descrip- 

 tion was rather incomplete and unaccompanied by illustrations 

 of any kind, besides being published in an obscure place ; but 

 the common occurrence of the species at the typical locality 

 leaves no room for reasonable doubt as to the shell Hall had 



