6 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 
Nautilide, 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 /o4e 
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 C/ymenide 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 Ammonitide 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-bearmg 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 MVautilus Pompilius, 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 fesfaceous 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 
Ammonitide, the Clymenide, and the Nautilide ; being ventral or external, that is, placed 
near the outer margin, in the Ammonitide ; central, that is, at or near the middle of 
the disc of the septum, in the NVautiide ; and dorsal, that is, close to the preceding 
volution, in the Clymenide. 
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 Wautili and the Ammonites, 
and their various cognate genera, were formed in the same way. In the recent 
