﻿BRITISH FOSSIL CEPHALOPODA. 



47 



Suborder Nautiloidea 

 (having a variable Siphuncle). 



Group I. Conici. — Cur- 

 vature slight or none; 

 form conical and re- 

 gular. 



I 



Shell straight : 

 Orthoceras. 



Shell curved : 

 Cyrtoceras. 



i 



Group II. Inflati. — Cur- ' 

 vature slight or none ; J 

 form inflated and ir- 

 regular. 



Group III. Spirales. — 

 Curvature consider- 

 able ; form simple. 



Group IV. Irregulares. 

 — Curvature consi- 

 derable but variable ; 

 form irregular or un- 

 symmetrical. 



Siphuncle very large and lateral : Endoceras. 



Siphuncle complicated : Actinoceras. 



Septa with a deep depression : Tretoceras. 



Septa bent forward over the siphuncle : Conoceras. 



Section fusiform : Gonioceras. 



Section triangular : Trigonoceras. 



(?) Siphuncle with conical sheaths : Piloceras. 



Shell slightly curved, aperture simple : Poterioceras. 



Shell straight and septa simple, aperture contracted : Gomphoceras. 



Shell curved and septa simple, aperture contracted : Phragmoceras. 



Later septa distorted : Ascoceras. 



Also with aperture partially contracted : Glossoceras. 



f Siphuncle internal, in a depression: Trocholites. 



Also with more or less angular sutures and whorls 

 uncovered : Clymenia. 



Also with involute whorls : Aturia. 



Whorls uncovered : Discites. 

 L With peculiar siphuncle : Nothoceras. 



Unsymmetrical : Trochoceras. 



Curvature lost in the body-chamber ; whorls out of contact [ ? ] : 



Lituites. 

 Whorls in contact and with a complex aperture : Opliidioceras. 

 Curvature changing ; body-chamber inflated : Cryptoceras. 



Whorls in contact 

 Nautilus. 



Whorls out of con- 

 tact : Gyroceras. 



It will be observed that the contraction of the aperture goes for very little in this 

 arrangement, though its importance in some cases is acknowledged. The reason 

 of this is that it seems possible for the aperture to be contracted without making 

 any other notable change in the animal, as when a Nautilus becomes a Hercoceras 

 of Barrande, a Trochoceras becomes an Adelphoceras, a Lituunculus of Barrande 

 becomes a Lituites, or an Ascoceras becomes a Glossoceras. In these cases the con- 

 tracted aperture appears to be of little importance compared with the other features. 

 On the other hand, Phragmoceras and Gomphoceras differ from the corresponding 

 forms Cyrtoceras and Orthoceras in the general structure of the shell, independently 

 of the aperture, which thus may be omitted from the definition. 



The value that should be placed on such curious deviations from the usual shape 

 of the septa in the region of the siphuncle, as are seen in the rare specimens named 

 by Barrande Nothoceras and Bathmoceras, must almost necessarily be a matter of mere 

 individual opinion. I am not disposed to place much weight on it, as Nature herself 

 does not seem to have done so, but rather to reverse the argument, and to learn from 

 these exceptions that the direction of the neck is not a matter of supreme importance. 



