﻿BEITISH FOSSIL CEPHALOPODA. 27 



the ornaments on the adult shell ; and, when observable, are available for specific 

 distinction. 



Another important matter connected with the commencement of the shell is its 

 position on the curve of motion. Were we to trace back this curve to the pole, we 

 should of course come to an indefinitely small shell ; but the actual nucleus is of 

 some considerable size at starting ; the effect of which is, that there is a vacuity in 

 the centre of every coiled shell, which may be of large size, or may be more or less 

 filled up by the irregularly shaped nucleus. In the Ammonitoidea the latter is the 

 case ; they commence very early in the curve, and the small initial vacuity is, in 

 most cases, nearly filled by their globular nucleus. In the Nautiloidea, on the 

 contrary, the vacuity is much more variable ; in the more involute forms it is often 

 small, and may be covered up by subsequent deposits, as in the Nautilus pompilius ; 

 but in the open-whorled groups, as the Lituites and Discites, there is sometimes a 

 vacuity of as much as one-fourth the whole diameter. This difference sometimes 

 serves to distinguish forms belonging to the two groups, which are externally much 

 alike. On this also depends the number of the whorls in the adult. Considering 

 that the growth of a Cephalopod is continuous, and consequently that every 

 specimen with many whorls must have previously had fewer, I was at one time 

 disposed to attach little importance to the number as a specific character ; but, all 

 things being equal, the greater the number of whorls in a given diameter, the earlier 

 must the shell have begun : used, therefore, with caution, the number of whorls is a 

 character of importance, as indicative of the relative size of the embryo. 



4. The Body Chamber. 



On the dimensions of the body-chamber depends the actual form of the animal, 

 and therefore, a priori, it should be of considerable consequence ; accordingly, in 

 recent attempts at the subdivision of the Ammonites, its length compared to that of 

 a whorl of the shell has been taken as one of the distinctive characters of families. 1 

 How far particular lengths of body-chamber characterise groups of Ammonites which 

 are united by other features will be considered at a future time ; but as far as the 

 Palaeozoic Nautiloids are concerned, the great variation in this respect which is 

 found in groups, such as Orthocerata and Trochocerata, united in all other features, 

 proves it to be of no value as a generic character, and of very doubtful value even 

 as a specific one. In Barrande's Etudes g enemies sur les Cephalopodes, chapter iv., 

 are given many details of the size of the body-chamber of all Cephalopods that have 

 come under his observation, and the proof of the variability will be found there. 

 In measuring the size of the body-chamber, the ratio of its length to its breadth is 

 most convenient in uncoiled shells, while in coiled ones the ratio of the length to one 



1 Suess, "Ueber Ammoniten," Sitz. K. AJcad. Wiss. Wien, vol. Hi. 1865; and Waagen, 'Die For- 

 menreihe des Ammonites subradiatus ;' Benecke's 'Beitrage,' Band ii. Heft 2, 1869. 



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