CEPHALOPODA. 
409 
Nautilus subliratus. 
PLATE LVII, FIGS. 4, 6, 7, and 5? 
Gt/roceras subliratum, Hall. Illustrations of Devonian Fossils: Cephalopoda, pi. 58, figs. 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 
armies 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. Siphitncle 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 the 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 strife, and on the lateral and ventral sides by distant, longitu¬ 
dinal striae. 
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 strife, as well as 
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