18 A POPULAR EXPOSITION OF THE 
ing water when pursued or otherwise alarmed. A single genus, the 
Argonaut, inhabits a one-chambered shell. All the other genera are 
“naked,” or without external shells, as seen in the Cuttle-fish. 
These dibranchiate cephalopods exhibit the higher organization, and 
approximate in some respects to the class of Fishes. Our Canadian 
rocks offer, however, no fossil representatives of this group, so 
abundant in existing Nature, and also to some extent so character- 
istie of the Mesozoic periods of the Harth’s history. The tetra- 
branchiate cephalopods, on the other hand, are almost extinct. The 
Nautilus is the only remaining type; and of this, no more than two 
or three living species are known; whilst from rocks of various ages, 
upwards of 150 fossil species have been collected. 
The shell in the tetrabranchiate group is divided into a number of 
compartments or chambers, by concave, sinuous, angulated, or highly- 
lobed partitions, called “‘septa””—the animal inhabiting the outer 
chamber—and it is traversed, throughout its entire length, by a tube 
or “siphuncle” of variable form and position. In the Nautilus, 
according to Professor Owen, this siphuncle opens into the cavity 
which contains the heart; and its use, although still doubtful, is 
thought to be to keep up the vitality of the shell in parts distant 
from the ereature’s body. It passes through the various chambers 
without affording any communication between these, so that the old 
idea respecting the use of the tube, and according to which the 
animal was thought by its means to be able to fill the chamber 
with water, or to eject this, in order to sink or rise at will, is 
now altogether exploded. Under ordinary conditions the nautilus 
appears to creep on the sea-bed, head downwards, at moderate depths, 
and to feed on holothuriz, star-fishes, erustacea, &c. The accom- 
panying diagrams, fig. 126, exhibit the marginal outline of the more 
general kinds of septa presented by the shells 
— — a of this group. A simple septum of the or- 
thoceratites and nautilus are represented by a; 
YAWN b an angulated septum of the goniatites by b ; 
m Soyo “eee lobed and denticulated septum of the cera- 
ti:es by c; and a foliated septum of the 
Mee g ammonites, baculites, hamites, &c., by d. 
“4 sah : In accordance chiefly with these characters, 
Fig. 126. the Tetrabranchiata, or chambered cephalo- 
pods, may be classed as follows : 
