532 PALEONTOLOGY OF ILLINOIS. 



Nautilus (Ceyptoceeas) capax, M. and W. 



PL 33, Fig. 1. 

 Nautilus Cryptoceras) capax, Meek and Worthen, 1865. Proceed. Acad. Nat. Sci., Phila., p. 262. 



Shell attaining a moderately large size, subglobose in form. Umbili- 

 cus deep, with abruptly sloping walls — one third as wide as the dorso- 

 ventral diameter of the outer whorl, and showing each of the inner 

 turns. Whorls about two and half, increasing rapidly in size, particu- 

 larly in transverse breadth ; last one so expanded laterally as to be 

 apparently one-third to one-half wider than its dorso-ventral diameter; 

 inner ones proportionally narrower. All broadly rounded on the outer 

 side, and more narrowly rounded with a flattened or slightly concave 

 revolving space between a ridge bounding the umbilicus and the middle 

 of each side; each provided with a narrow, shallow impression along 

 the ventral side for the reception of the inner volutions. Septa sepa- 

 rated by spaces which measure on the outer side less than one-fourth 

 the dorso-ventral diameter of the volution at the point of measurement 

 a little arched backwards on the slightly concave inner side of the 

 whorls, and less distinctly so on the narrow revolving flattened space 

 just outside of the umbilicus, after which they cross over the broadly 

 rounded outer side, with a very low scarcely perceptible backward 

 curve. Aperture transversely oval, or subelliptic, and apparently angu- 

 lar, and effuse at each inner lateral margin. Outer chamber very capa- 

 cious, composing less than half a volution. 



Greatest diameter across the disc about 7 inches ; breadth (transverse 

 diameter of the aperture) 6 inches; dorso-ventral diameter 3.25 inches, 

 breadth of umbilicus 1 inch. 



The only specimen of this species we have seen is a cast, which shows 

 along the outer side of the whorls the appearance of a tube 0.20 inch in 

 diameter, extending backwards from each septum. It is possible this 

 may be a small lobe of the septa, but we have scarcely a doubt in regard 

 to its being the siphon, and hence that the species belongs to the group 

 Cryptoceras. 



Compared with N. dorsalis, Phillips, (Geol. Yorks. II, PI. 13, fig. 1 

 and 2,) the type of the group Cryptoceras, our shell will be found to 

 differ in its much more broadly rounded dorsum, and much wider mouth, 

 as well as in the peculiar revolving flattened space near the umbilical 

 side of the whorls, which imparts a slight angularity to the margin of 

 the umbilicus, as well as an undefined longitudinal ridge or prominence 

 near the middle of the whorls on each side. 



Position and locality : Charboniere, Missouri. Coal Measures. 



