CEPHALOPODA. 409 



Nautilus subliratus. 



PLATE LVII, FIGS. 4, 6, 7, and 5? 

 Oi/roceras subliratum, Hall. Illustrations of Devonian Fossils : Cephalopoda, pi. 58, fig's. 1, 2. 1876. 



Shell subglobose or subovoid, ventricose, making more than a single volution. 

 Volutions contiguous, not embracing; umbilicus exposing all the whorls. 

 Transverse section broadly elliptical, usually flattened from compression, and 

 with a gentle sinus on the concave dorsal side. Tube rapidly expanding. 

 Apical angle, as measured from a compressed specimen, about 19°. 



Chamber of habitation having a length about twice the diameter at the 

 base, which is two-thirds the apertural diameter. Aperture opening at right 

 angles to the direction of the axis of the tube; the dorsal and dorso-lateral 

 margins plicated. Air-chambers regular, the last one or two shallower than 

 the preceding. 



Septa regular, making a slight inflexion over the lateral face of the volu- 

 tion ; distant on the convex side of the outer volution from six to eight or 

 nine mm., and less than half that distance on the concave side. Suture lines 

 distinctly marked in the cast. Siphuncle subcentral. 



Test not preserved, except some portions of the inner laminae in a macer- 

 ated and exsolute condition. Surface on the concave dorsal side, and par- 

 tially on tho lateral faces, marked by strong, gradually diverging, rounded, 

 revolving ridges or plications, with wider, concave interspaces, which are 

 usually absent along the median area, and become gradually obsolete, or 

 abruptly disappear toward the ventral side. Entire surface marked by regu- 

 lar, transverse stria), and on the lateral and ventral sides by distant, longitu- 

 dinal stria). 



In a cast of the grand chamber of a specimen in which scarcely any 

 surface-markings remain, there are two broad, shallow, undefined, longitu- 

 dinal depressions, traversing the entire ventral side of the chamber, while 

 the margin of the aperture shows obscure, undefined sinuosities. 



The internal cast is marked by the impressed septal lines, the strong 

 radiating ridges, and the fine transverse and longitudinal striae, as well as 

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