T12 HISTORICAL PALZONTOLOGY. 
which it is organically connected by muscular attachments. 
The head is furnished with long muscular processes or ‘‘ arms,” 
Fig. 54.— Different views of laclurca crenulata, Quebec Group, Newfoundland. 
(After Billings.) : 
and can be protruded from the mouth of the shell at will, or 
again withdrawn within it. We learn, also, from the Pearly 
Nautilus, that these animals must have possessed two pairs of 
breathing organs or ‘gills ;” hence all these forms are grouped 
together under the name of the “ Tetrabranchiate ” Cephalo- 
pods (Gr. ¢etra, four; bragchia, gill). On the other hand, the 
ordinary Cuttle-fishes and Calamaries either possess an internal 
skeleton, or if they have an external shell, it is not chambered ; 
their “arms” are furnished with powerful organs of adhesion 
in the form of suckers ; and they possess only a single pair of 
gills. For this last reason they are termed the ‘“ Dibranchiate ” 
Cephalopods (Gr. dis, twice ; bragchia, gill). No trace of the 
true Cuttle-fishes has yet been found in Lower Silurian deposits; 
but the Tetrabranchiate group is represented by a great num- 
ber of forms, sometimes of great size. The principal Lower 
Silurian genus is the well-known and widely-distributed O7t/o- 
ceras (fig. 55). The shell in this genus agrees with that of the 
existing Pearly Nautilus, in consisting of numerous chambers 
separated by shelly partitions (or septa), the latter being per- 
forated by a tube which runs the whole length of the shell 
after the last chamber, and is known as the “siphuncle” (fig. 
56, s). The last chamber formed is the largest, and in it the 
animal lives. The chambers behind this are apparently filled 
with some gas secreted by the animal itself; and these are sup- 
posed to act as a kind of float, enabling the creature to move 
with ease under the weight of its shell. The various air- 
chambers, though the sipbuncle passes through them, have no 
direct connection with one another ; and it is believed that the 
animal has the power of slightly altering its specific gravity, 
and thus ef rising or sinking in the water by driving additional 
fluid into the siphuncle or partially emptying it. The Ortho- 
