﻿24 BRITISH FOSSIL CEPHALOPODA. 



the Gastropods and Spirula takes place in one direction, so that the dorsal is the 

 convex side, and that of the Nautilus in the opposite, the dorsal being the concave 

 side, we may be prepared to find in the less settled Palseozoic forms variations in 

 this respect in more closely allied groups. For these two directions the terms 

 "endogastric " and " exogastric " were proposed by Ssemann in 1852/^6 former 

 for those in which the ventral side of the animal was on the concave side of the shell, 

 and the latter for such shells as the Nautilus. It is not easy to determine which was 

 the ventral side in these fossil shells. Since in the recent Nautilus the exterior has 

 a backward sinus which is indicated by the parallel lines of growth, it is assumed 

 that such a sinus in the ornaments of a fossil indicates its ventral side. Cases, how- 

 ever, are not wanting in which a sinus occurs on both sides of the shell, as in the 

 Phragmocerata ; hence we have no grounds for assuming in coiled shells that the 

 true sinus for the ventral side was not internal. It is also assumed, and no doubt 

 truly, that the small aperture of Gomphocerata, &c, indicates the ventral side; but 

 whether this be so or not, it must always indicate the same side of the animal ; and 

 this affords conclusive proof that in this group, at least, there was sometimes a 

 curvature in one direction and sometimes in the other ; for in Phragmoceras this 

 aperture is more usually, but not constantly, on the more concave side, while in 

 Gomphoceras it is more commonly on the convex side. 



3. The Commencement. 



When the shell of a Nautiloid is traced towards its smaller end, a point is reached 

 at which it ceases to have the conical shape of its adult age, and the apex of the cone 

 is replaced by a cap of some other shape. This cap, besides its shape, often shows 

 other peculiarities which distinguish it from the rest of the shell. The importance 

 of the study of this part is obvious from its connection with the development of the 

 Nautiloid. We may here learn those characters which point to the origin of the 

 forms possessing them, and any fundamental distinction found will prove a bifurca- 

 tion of the group. In point of fact the Nautiloidea and Ammonitoidea are as much 

 separated by the characters of this part as they are by those of the adult. So 

 important from a theoretical point of view has M. Barrande considered this, that of 

 all his large work containing general observations on Cephalopods he has selected 

 the part containing his observations on it to be separately printed and distributed, as 

 his most potent proof of the falsity of the doctrine of Evolution. In the Ammoni- 

 toidea, as far as yet ascertained by accurate observations, the cap is always inflated 

 and more or less globular in form, and has a smooth exterior. From one side of it 

 the shell starts at once in its regular conical form. This is the case even with those 

 G-oniatites of the Silurian rocks, in which the commencement has been observed. 



No such inflated cap is to be found in any of the Nautiloidea, so that in this 



' J Palseontographica.' 



