﻿28 BRITISH FOSSIL CEPHALOPODA. 



whorl length is better. Of course, when the other elements of the shell are given, 

 these are mutually convertible, and from either we can find by a simple formula the 

 capacity of the body-chamber to that of the whole shell, which latter ratio is no new 

 element unless there are abnormal inflations or contractions. 



If the length wants constancy, it is otherwise with the shape. The grouping of 

 united features is such that any peculiarity in the latter is indicative of several 

 others in independent parts of the organism, and consequently I am led to consider 

 the shape of the body-chamber as a generic distinction. Thus in the genera 

 Orthoceras, Cyrtoceras, Nautilus, &c, we have but little change from the earlier shape 

 of the shell. In Phragmoceras and its allies we have a peculiarly inflated body 

 chamber ; and in Lituites we have the curvature lost there ; while in Ascoceras and 

 Tretoceras we find it encroaching abnormally on the septal portion. In the study, 

 therefore, of fossil Cephalopods, the peculiarities shown in this part of the organism 

 are of the highest importance. 



5. The Aperture. 



There are two distinct kinds of aperture among the Nautiloidea, the simple and 

 the contracted. In the simple kind, whatever may be the outline in a transverse 

 direction, the surface of the shell does not bend in towards the centre of the section; 

 in the contracted kind this bending in does take place, and produces a variously 

 formed aperture having quite a different outline to that of the section of the shell. 

 The latter kind is comparatively rare among the Ammonitoidea, and it is chiefly in the 

 suborder we are now concerned with that the distinction is of importance. Barrande 

 places the form of aperture in the first rank of characters, dividing all the Nautiloids 

 into two series by means of it, and complains that other Palaeontologists have paid 

 too little attention to it in their classifications. Yet I can see no reason for treating 

 it in any different manner from the rest, or for not judging of its importance by the 

 other characters found constantly associated with it. Among Gastropods, doubtless, 

 the form of the aperture is much used in classification ; but this is only when it has 

 some proved relation to the animal ; for examples are known among the Helicidce 

 in which a similar lessening of the aperture takes place, without entailing any other 

 changes which are sufficient to separate the animal generically. 



The most ordinary form of simple aperture is that in which the sides of the shell 

 are produced into a convex lobe, leaving the dorsal and ventral borders as sinuses 

 between them. The recent Nautilus is so shaped, and we know that in it the 

 ventral sinus corresponds to the position of the funnel. That side of the shell in 

 extinct Nautiloids which has the most marked and constant sinus may be assumed 

 with probability to be the ventral side. The amount to which these side lobes are 

 produced, and the corresponding depth of the sinus, varies exceedingly. Among the 

 Ammonitoidea it is not uncommon to find long tongue-shaped productions ; but 





