41G PALJBOXTOLOQr OF NEW YORK. 



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 dor.«al 

 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 mm., 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 stria?, 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 stria?. 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 lamina) 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. 



