﻿4 BRITISH FOSSIL CEPHALOPODA. 



upper, side of the body. In some cases the curvature of the shell has also followed 9 

 being coiled in opposite directions in a Nautilus and in a snail ; but the Spirula is 

 an exception to this rule. 



These are the chief points in which the Cephalopoda, as a class, differ from the 

 rest of the Glossophora ; other distinctions which are either matters of detail, or 

 refer to structures so peculiar that no comparison holds, will be seen from the de- 

 scription of the anatomy of each main type of the class, as given in the subsequent 

 pages. From their similarity in the matter of the foot and of the flexure of the 

 intestine, as well as in the large development of the epipodia, which become in the 

 Pteropods the sails and in the Cephalopods the funnel, the latter are more nearly 

 allied to the former than to the Gastropods. Whether the development of the 

 Cephalopoda has been through the Pteropoda, or whether they both branched off 

 from an early form of Gastropod, or even Lamellibranch, we have not, as yet, 

 especially in the absence of all knowledge of the Nautilus development, sufficient 

 data to decide. The similarity of the young Pteropod to a young Gastropod 

 suggests the second supposition as the most probable one. Traced back to the 

 earliest formations in which they occur, the Pteropods antedate all the other Mol- 

 lusca ; the Heteropods come next ; the Cephalopods and Lamellibranchs are found 

 first in the same rocks, and the Gastropods come last of all. Little importance can 

 be attached to this order of appearance, as it is liable to be corrected at any moment, 

 and affords no proof of the descent of one class from another ; it simply adds to the 

 balance of probability that the Cephalopods came through the Pteropods, and 

 perhaps also through the Heteropods. 



At the time of its foundation the class Cephalopoda contained but one order, in 

 which the Nautilus was placed with the rest ; but from the date of Professor Owen's 

 Memoir on that animal, in the year 1832, in which he showed the important differences 

 in its organisation, as compared with all others of the class, there has never been 

 any doubt as to the propriety of recognising two orders — the Dibranchiata for all 

 previously known forms, and the Tetrabranchiata for the genus Nautilus alone 

 among living animals. 1 Although the two orders are thus apparently very unequal, 

 yet when the fossil forms are included the proportion is reversed, as the greater 

 number of the latter belong to the second order. In point of fact, the whole of the 

 Silurian Cephalopoda, and nearly all the rest of the Palaeozoic ones, are believed to 

 have been tetrabranchiate. With these then alone we have at present to deal, and 

 their description will be prefaced by a detailed account of the anatomy of the 

 Nautilus, as made known to us by the researches of Owen, Yalenciennes, Yrolik, 

 Macdonald, Huxley, Yan der Hoeven, and Keferstein. 



1 Different names have been assigned to these orders by D'Orbigny, contrary to the rights of 

 priority— viz., Acetabulifera and Tentaculifera — but the limits of the two orders are the same, 





