306 TALiEONTOLOGY OF ILLINOIS. 



tion; septa rather deeply concave, with outer margins nearly 

 straight in passing over the dorsum, where they are separated 

 by spaces a little less than one-fifth their own greater transverse 

 diameter; aperture transversely lunate or reniform; lip deeply 

 sinuous on the dorsal side (not represented quite deep enough 

 in our figure); siphuncle small, central. (Surface unknown.) 

 Greatest diameter, 2.11 inches; height, 1.72 inches; breadth 

 of aperture, 2.06 inches. 



At the time we proposed the name N. subglobosus for this form, we had not 

 access to Sowerby's figures of his N. globatus, but made our comparisons with 

 Prof, de Koninck's figures of a Belgian shell referred by him to Sowerby's 

 species, (Anim. Foss. Carb. Belg., pi. xlvii, fig. 11 a and 11 Z».) From the form 

 represented by Prof, de Koninck, our shell differs so materially, that we could 

 have no doubt in regard to its being specifically distinct, as it expands much 

 more rapidly towards the aperture, and has a smaller umbilicus; while it shows 

 no traces of the transverse ridges or costae, forming so conspicuous a feature in 

 the figures given by Prof, de Koninck. On comparing our shell with Sowerby's 

 original figures, however, in connection with Prof. McCoy's description of N. 

 globatus, we can scarcely doubt the propriety of referring it to that species. 

 The only essential difference seems to be in the size of the siphon, which in 

 our shell is small, while McCoy describes it as being large in N. globatus The 

 difference in this character may not, however, be so great as to constitute a relia- 

 ble specific distinction. 



Locality and position : Chester, Illinois; Chester group of the Subcarbonif- 

 erous series. 



Nautilus chestepensis, M. and "W. 



PI. 24, fig. ia, 4 b. 



Nautilus Chester ensis, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. Sci., 

 Philad., p. 469. 



Shell under medium size, subglobose, rounded over the 

 dorsum and sides; umbilicus (in casts) small and deep, with 

 nearly vertical walls, probably almost closed in adult speci- 

 mens retaining the shell, apparently showing very little of the 

 inner whorls; volutions about three, increasing moderately in 



