416 
PALAEONTOLOGY OF NEW YORE. 
Nautilus cavus, n. sp. 
PLATE CVI, FIGS. 1-3. 
Shell subglobose, ventricose. Volutions at least two; the inner ones re-entrant 
and exposing only the lateral faces. Umbilicus deep. Transverse section 
subelliptical, somewhat concave, and indented in the middle of the dorsal 
side by a deep sinus produced by the inner adjacent volution. Transverse 
diameter nearly twice as great as the dorso-ventral diameter. Tube rapidly 
expanding. Apical angle about 16°. 
Chamber of habitation and aperture unknown. Air-chambers regular, 
comparatively deep, and increasing in depth with the expansion of the 
volutions. 
Septa regular, distant, increasing in distance from eight to fifteen mm. in 
the space of less than half a volution. The septa, at their origin on the con¬ 
cave-dorsal side, are distant about two mm., and make an abrupt retral 
curve in the sinus. A length of eighty min., measured on the convex curve 
of the outer volution, preserves seven septa and six air-chambers entire. 
The suture lines are conspicuously marked on the cast. Siphuncle undeter¬ 
mined. 
Test not preserved. The surface has been marked by lamellose lines of 
growth and regular revolving striae, as indicated by a small fragment of the 
macerated test remaining on the cast. A deep sinus marks the concave 
dorsal side. 
The cast of the interior is essentially smooth, except the impressed suture 
lines, and some remains of the impression of the surface striae. A narrow 
raised line is preserved on the convex ventral side of the air-chambers, indi¬ 
cating the ventral carina. Where the test is entirely removed the cast 
appears smooth; but where the inner laminae remains the surface is covered 
by a fine mammillary deposit. 
A fragment preserving eight septa has a length of about ninety mm., 
as measured on the convex curve; its greatest transverse diameter being 
about fifty-five mm. 
