36 OBSERVATIONS ON THE ANIMALS 
pounce) is apparently solid, though, even in this case, it is made up 
of plates of carbonate of lime intersecting one another, and forming 
an indefinite number of minute cells filled with air; thus making an 
apparently stony substance really light and friable. In others, again, 
as the minute internal shells of the microscopic cephalopods, we find 
the cellular structure more clearly developed, and forming a succes- 
sion of chambers very numerous, indeed, but not innumerable. In 
these there is a connection between the chambers by a simple aper- 
ture, and their use seems to be shown by their approximation to bony 
structure giving a solid frame-work, to which the muscles can be 
attached ; and at the same time, by the quantity of air in the cham- 
bers reducing the specific gravity of the whole animal, so as to allow 
of its floating near the surface, or at a proper depth of water. 
Lastly, we have a shell constructed like a Nautilus, in which there 
is a succession of chambers, with a tube running through all of them, 
and passing into the animal itself, which in this case lives entirely in 
the outer chamber, and as it increases in size continually enlarges its 
house, and adds another to the empty rooms which it once inhabited. 
Now it must be observed that the highest in order of organization 
of all these cephalopods, is that group with an internal cellular skele- 
ton like the Cuttle-fish ; and this is indicated by the way in which 
this skeleton is put together. It consists of numerous nearly flat 
layers placed within each other, the first formed being at the outer 
part and posterior termination of the shell, and the succeeding new 
layers extending always more forwards than the edges of the old. 
These compressed layers are connected together by numerous very 
minute tubular fibres ; so that in this internal laminated shell there is 
a structure intermediate between that of the external multilocular 
shells of the less highly organized genera, and that fibrous cellular 
substance which is called bone in animals of higher organization. 
It is not, however, with remains referred to this group that the 
geologist has most todo. They are useful to him, indeed, in the 
way of analogy, but either they did not exist in the ancient seas, or 
the friable skeleton was not of a nature to be easily preserved under 
ordinary circumstances. It is the shells resembling Nautilus, such as 
Ammonite and its congeners, whose abundant remains are presently 
to come under our consideration. 
But first it may be worth while to inquire how it is that animals 
so high in their organization, and once so very numerous as the Ce- 
phalopods undoubtedly were, could have vanished so entirely as they 
seem to have done, without leaving some hiatus in nature. There is, 
a 
