CEPHALOPODA. 
425 
NAUTILUS, Subgenus Discites, McCoy. 
The three following species possess characters which, in many respects, are 
very unlike the preceding. The general form of the shell and position of the 
siphunele, as well as the surface-markings and ornamentation, are quite distinc¬ 
tive. They are all laterally compressed and angular, or subangular, on the 
umbilical and peripheral margins. The septa make a distinct, more or less 
deep and angular sinuosity on the dorso-lateral and ventro-lateral angles of the 
volutions, precisely as in the simpler forms of Goniatites. 
For forms of this character Prof. McCoy proposed the name Discites, which 
however, has been but partially recognized by authors, the species being 
usually arranged with Nautilus proper. It appears to me that possessing so 
many characters, which are widely different from the typical forms of that 
genus, they should receive some distinct recognition, at least as a subgeneric 
type. 
Nautilus (Discites) ammonis, n. sp. 
PLATE—SUPPLEMENT. 
Shell large, discoid; the greatest thickness of the disc is equal to one-seventh 
of its diameter, or one-half the dorso-ventral diameter of the outer volution. 
Volutions about three, having the dorso-ventral diameter greater than the 
transverse diameter. Inner volutions re-entrant for about one-seventh of 
their diameter. Umbilicus very wide, exposing the lateral faces of all the 
volutions. Tube regularly and very gradually enlarging, the sides of the 
dorsal and ventral faces diverging at an angle of 10°. Transverse section 
subangularly elliptical. 
Chamber of habitation very large, slightly expanding toward the aperture; 
occupying more than one-third of the outer volution. Aperture apparently 
opening a little outward. Air-chambers numerous, regular, gradually 
increasing in depth toward the base of the grand chamber; about four in 
the space of forty-five mm. 
54 
