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 

 siph uncle, 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— SUPPLtMK.NT. 



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 

 pubangularly 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. 

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