006 PALAEONTOLOGY OF ILLINOIS. 



a broad depressed-subconical umbilicus. Surface marked 

 with rather strongly denned lines of growth, which, at 

 places, become sub-imbricating, or form little irregular 

 ridges. In crossing the upper side of the whorls, these 

 lines start, at first, nearly at right angles from the sutures, 

 but curve a little backward as they approach the marginal 

 angle; and immediately after crossing this angle, and pass- 

 ing downward upon the nearly vertically flattened peri- 

 phery, they are deflected a little forward, but soon after 

 pass straight down to and over the lower marginal angle 

 to the under side, where they extend obliquely backward 

 and inward, with a rather distinct curve, to the immediate 

 vicinity of the suture, and then curve a little forward. 

 Aperture and sections of interior of whorls subcircular, or 

 transversely oval. 



Greatest breadth of a specimen with apparently about 

 one-third of the outer volution broken away, 1.25 inches; 

 hight, 0.50 inch. When entire this specimen was probably 

 not less than 1.43 inches in breadth. 



From the foregoing description it will be seen that this shell is nearly 

 allied to the common Western Coal Measure species figured and de- 

 scribed by Prof. HALL, in his Iowa Geological lieport, under the name 

 Euomphalus rugosits, (not E. ruf/owix, of Sowerby.*) Indeed it is so 

 nearly like that species that we at one time suspected that it migMt be 

 only a gigantic and more ventricose variety of the same. Yet on com- 

 paring our shell with an extensive series of good specimens of E. ruijo- 

 SUK, Hall, it is found to be greatly larger than any known authentic 

 examples of that shell, its breadth being a little more than twice and a 

 half that of the usual mature examples of E, rugosus. Its umbilicus, 

 and the concavity of its upper side, are also proportionally deeper, par- 

 ticularly the former, while the flattened outer side of its whorls is 

 broader, and generally less oblique. Its lines of growth also differ in 

 being rather distinctly deflected backward at the marginal angle of the 

 upper side, so as to indicate an obscure sinus of the margin of the lip 

 at the termination of this angle, though there are no traces of a band, 

 as in Pleurotomaria. This character would probably place the species 



* If SOWEKBY'H species is really congeneric, the American form culled E. ruguum, by Prof. HALL, 

 might be distinguished by the specific iiauie subrwjomis. 



