NATURAL SCIENCES OP PHILADELPHIA. 261 



in form than those of more modern date, we have concluded to place it for the 

 present, as a variety of N. ornatus. 



To facilitate compxrison, we would remark that our shell, when entire, 

 must have measured from 10 to 12 inches in its greatest diameter across the 

 disc ; while its outer whorl measures about 5 inches in thickness or convexity 

 at the umbilical side, and 1-50 inches at the periphery. The greatest breadth 

 of the sides of the outer turn is about 4*50 inches. Its septa, as seen on each 

 side, are arranged and curved very nearly as in the typical form of N. ornatus, 

 and are deflected, backwards on the truncated periphery, so as to form a sub- 

 trigonal lobe as deep as wide ; they also make a rather broad, deep, back- 

 ward curve on the inner side- Surface markings unknown. 



The shells of this type seem to be very distinct from the typical recent 

 forms of Nautilus, and whatever others may think, should, as we believe, be 

 at least regarded as forming a marked subgeneric group. Objections have 

 been raised to the use of McCoy's name, Discites, however, because De Haan 

 had used it in 1825 for a type of Nautilidas ; but as De Haan merely used it in a 

 kind of synoptical table, with no other characters than " Sensim incrq/satis," 

 without a figure, or any allusion to any type by which the most remote con- 

 jecture can be formed iu regard to what group of Nautilidae he meant, it must 

 be evident we cannot regard him as having established either a genus or a 

 subgenus, and the name was consequently free to be used by any other author. 



Locality and position. Hamilton Group (Devonian) ; " Devil's Bake-oven, " 

 Jackson County, Illinois. 



Nautilus (Discites) discifoiuii-, M. & W. 



Shell attaining a rather large size, discoid, much compressed ; umbilicus 

 shallow, a little wider than the dorso-ventral diameter of the outer volution, 

 and showing all the inner turns. Whorls about three and a half, nearly con- 

 tiguous, or very slightly embracing, nearly flat on each side, the greatest con- 

 vexity being about half way between the middle and the inner side, from 

 which point the sides round into the umbilicus, and converge to the periphery, 

 which is truncated, narrow and concave. Septa rather closely arranged, 

 crossing the sides of the whorls with a broad, graceful backward curve and 

 more abruptly flexed in the same direction on the truncated periphery, so 

 as to form a subtrigonal lobe about as deep as wide ; also, somewhat curved 

 backwards on the inner side of the whorls. Last chamber' long, or forming 

 about half the outer volution ; others shallow, or usually about equalling one- 

 fifth the dorso-ventral diameter of the volutions at the point of measurement. 

 Siphon small, suboval, located about its own diameter outside of the middle of 

 the whorls. Aperture and section strongly compressed, subovate, the inner side 

 being rounded, and the outer, which is much narrower, truncated, somewhat 

 emarginate, and biangular. (Surface unknown.) 



Greatest diameter across the disc, SJ to 9 inches ; dorso-ventral diameter 

 of outer whorl nearly 3 inches; greatest convexity of whorls, about 1-50 inches ; 

 breadth of truncated periphery, 0*68 inch. 



In size, form, proportions, breadth of umbilicus, &c, as well as in the 

 arrangement aud curvature of its septa, this species seems to agree almost 

 exactly with the discus of Sowerby. If that species has been correctly figured 

 and described, however, our shell presents the important difference of having 

 its siphon located a little outside of the middle, instead of near the inner 

 margin of the whorls. 



Locality and position. Niota, Hancock County, Illinois. Keokuk Division 

 of Subca'bouiferous Series. 



Nautilus Lasallensis, M. & W. 



attaining a m 

 is more than 



I860] 



Shell attaining a medium size, compressed, subglobose, or subdiscoidal ; 

 umbilicus more than half as wide as the dorso-ventral diameter of the last 



