30 



Second Order. — Tetrabranchiata (Owen, 1832), containing only one 

 recent Genus, Nautilus. 



31. Nautilus, Breynius, 1732 



Body round, furnished with a considerable 

 number of arms, one pair of which are 

 expanded and form the hood to close the 

 aperture of the shell; arms retractile 

 into sheaths, without suckers ; fins 

 absent ; eyes large, on stalks. Shell 

 involute, with the later whorls partially 

 enveloping the earlier, many chambered, 

 each chamber perforated by a central 

 siphuncle, the exterior chamber con- 

 taining the body of the animal ; aperture 

 of last chamber regular, septa of cham- 

 bers without irregular folds. 



APPENDIX II. 



SCHEDULE OF THE FOSSIL CEPHALOPODA. 



Description of the Class. 



Animal generally unknown, allied either to the Argonauta, to the families 

 of the Decapoda, or to the Nautili. Shell, either internal or external ; 

 sometimes lance-shaped, generally chambered, and in form founded upon 

 the motion of a point revolving and advancing in space about a central 

 axis, the axis itself being straight or curved, or a combination of straight 

 lines and curves, in the same plane, or not in the same plane. Exterior 

 chamber generally large, with the outer edge open or contracted. Cham- 

 bers divided by partitions {Septa), with edges (Sutures) plain, slightly or 

 intricately undulating, with elevations (saddles), and depressions (lobes) ; 

 Chambers pierced by a small opening (leading into the Siphuncle), variable 

 in position. 



The fossil Cephalopoda are divided into families and genera, depending 

 upon the form of the shell, the undulations of the partitions (Septa), the 

 position of the small opening (Siphuncle), and the plan of the external 

 chamber, etc., and may be thus tabulated : — 



