186 GASTEROPODS. 



minences on the shell ; when the tubercles were arranged in 

 regular rows, there appeared a series of nodular plications. In 

 many cases the gasteropod shell increased in size much faster 

 than the echinoderm, and the lip of the shell consequently often 

 encompassed the two posterio-lateral arms, and not unfre- 

 quently, also, the stem of the crinoid. The result was two 

 large, deep sinuses in the anterior, and one similar indentation 

 in the posterior margin of the shell. The effect of the tubercles 

 was to impart a similar sinuous character to the entire margin, 

 hence the lip was always crenated during the latter part of the 

 mollusk's existence. The continual change in the nature of the 

 surface upon which the gasteropod rested, also interfered with 

 the uniform and regular growth along theapertural margin, and 

 the lines of growth are consequently often strongly imbricated. 



Igoceras fissurella ( HALL). 



Plate Hi, fig. 2. 



Platyceras fissurella Hall, ]859 : Geology Iowa, vol. I, Supp., p. 90. 

 Platyceras fissurella Meek & Worthen, 1873: Geol. !Sur. Illinois, vol. V, p. 



519, pi. xvii,fig. 4. 



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

 Capulus fissurella Keyes, 1890: Am. Geologist, vol. Vf, p. 9. 

 Capulus fissurella Keyes, 1890: Proc. Acad. Nat. Sci., Phila., p. 173. 



The species under consideration is closely allied to Igoceras 

 pabulocrinus (Owen), from which it differs chiefly in being 

 much more depressed, wifch the aperture consequently very 

 much larger in proportion to the size of the shell. 



Horizon and localities. Keokuk limestone : Warsaw and 

 Nauvoo, Illinois; Burlington limestone: Burlington (Iowa). 



The specific name of this form is very inappropriate, hav- 

 ing originated in a misconception on the part 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 and 

 Worthen that the aperture in the apex is not a natural opening, 

 but an accidental fracture in the shell. 



Recently typical examples of /. 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 



