522 PALAEONTOLOGY OF ILLINOIS. 



would seem to represent a more slender, or less rapidly expanding form, 

 with less arching transverse strise. 



Locality and position Keokuk division of the Lower Carboniferous 

 series, Hamilton, Illinois. 



CEPHALOPODA. 



GENUS NAUTILUS, Linn. 



NAUTILUS (DISCITES) DISCIFORMIS, M. and W. 



PI. 18, Fig. 1. 

 Nautilus (Discites) diseiformis, MEEK and WOUTHEN, 1865. Proceed. Acad. Nat. Sci., Phila., p. 261. 



SHELL attaining a rather large size, discoid, much coin- 

 pressed; 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 

 contiguous, or very slightly embracing, nearly flat on each 

 side, the greatest convexity 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 closely ar- 

 ranged, crossing the sides of the whorls with a broad, grace- 

 ful, backward curve, more abruptly flexed in the same di- 

 rection on the truncated periphery, so as to form there a 

 kind of subtrigonal lobe about as deep as wide ; they are 

 also sometimes a little curved backwards on the inner side 

 of the whorls. Last or body chamber long, or forming 

 about half the outer volution; others shallow, or usually 

 about equaling one-fifth the dorso-ventral diameter of the 

 volutions at the point of measurement. Siphuncle small, 

 suboval, located about its own diameter outside of the mid- 

 dle of the whorls. Aperture and section strongly com- 

 pressed, subovate, the inner side being rounded, and the 

 outer side, which is much narrower, truncated, somewhat 

 emarginate, and biangular. Surface unknown. 



Greatest diameter across the disc, 8.50 inches to 9 inches; 

 dorso-ventral of the outer volution, near 3 inches ; greatest 



