EOCENE MOLLUSCA. 



the last of which the body is contained. The partitions present the greatest variety 

 of form ; being in fact moulded upon the animal, they indicate corresponding zoological 

 peculiarities, and generic distinctions have been founded upon them. Among the 

 Nautilida, one of the families into which the tetrabranchiate Cephalopods are divided, 

 the posterior extremity of the body is round and without any projecting part, or lobe 

 as it is termed, and the septa therefore are characterised by simple curvatures or 

 undulations, and their margins are always entire ; and thus we are led by analogy to 

 believe, that in the Clymcnidce the animal had an angular lobe on each side of the body, 

 from which the sinus, which characterizes their septa, would take its form ; and that 

 in the Ammonitidcs the posterior extremity of the body had many lobes, the edges of 

 which were foliated, whence the septa assumed corresponding curvatures with foliated 

 margins. Sometimes, and this is most generally the case among the recent 

 Cephalopods, the animal is without the protection of an external shell ; but it is 

 then supplied either with a calcareous chambered shell almost wholly buried in the 

 animal, or with a horny or calcareous substance, simple, or more or less complicated in 

 form and structure, wholly internal, and encysted in the back of the mantle. From 

 the presence or absence of the external shell, the Cephalopods have been, and in fact 

 still are, popularly divided into shell-bearing and naked Cephalopods, although in the 

 systematic arrangement proposed by Professor Owen these terms have a more 

 restricted application. 



The chambered shells are characterised by a peculiar apparatus, by means of 

 which, as it has been generally supposed, they are made subservient to hydrostatic 

 purposes, although the precise mode by which that end is attained is merely conjectural. 

 From Professor Owen's description of the Nautilus Pompilitts, it appears that the 

 posterior part of the visceral sac is prolonged in the form of a membranous tube, 

 which, passing through a short calcareous collar, formed in the disc of each septum, 

 and called the testaceous siphon, traverses the different chambers to the extreme nucleus 

 of the shell. This tube, with the calcareous collar which, more or less, covers and 

 protects it, is termed the siphon or siphuncle, and is found in all the multilocular shells 

 strictly so called, whether external or internal, recent or fossil ; and its position with 

 reference to the margin of the shell, is used as another distinction between the 

 Ammonitida, the Cli/menida, and the NautilidcR ; being ventral or external, that is, placed 

 near the outer margin, in the Ammonitidse ; central, that is, at or near the middle of 

 the disc of the septum, in the Nautilida ; and dorsal, that is, close to the preceding- 

 volution, in the Clymenida. 



The process by which the external shells of the Cephalopods are constructed does 

 not appear to differ essentially from that used by the inferior molluscs. Professor 

 Owen has described the mode of growth in the Nautilus Pompilius ; and we are led 

 by analogy to the conclusion, that the shells of the extinct Nautili and the Ammonites, 

 and their various cognate genera, were formed in the same way. In the recent 



