Turtles are found on the sea and on land, the marine forms more properly
deserving the name of turtles; tortoises being those living on land or in
fresh water. We shall use the name turtle as significant of the whole
group. The most natural method of classifying these creatures is by the
way the head and neck are drawn back under the shell; whether the head is
turned to one side, or drawn straight back, bending the neck into the
letter S shape.

The skull of a turtle is massive, and some have thick, false roofs on top
of the usual brain box.

The "house" or shell of a turtle is made up of separate pieces of bone, a
central row along the back and others arranged around on both sides. These
are really pieces of the skin of the back changed to bone. Our ribs are
directly under the skin of the back, and if this skin should harden into a
bone-like substance, the ribs would lie flat against it, and this is the
case with the ribs of turtles. So when we marvel that the ribs of a turtle
are on the outside of its body, a second thought will show us that this is
just as true of us as it is of these reptiles.

This hardening of the skin has brought about some interesting changes in
the body of the turtle. In all the higher animals, from fishes up to man,
a backbone is of the greatest importance not only in carrying the nerves
and blood-vessels, but in supporting the entire body. In turtles alone,
the string of vertebrae is unnecessary, the shell giving all the support
needed. So, as Nature seldom allows unused tissues or organs to remain,
these bones along the back become, in many species, reduced to a mere
thread.

The pieces of bone or horn which go to make up the shell, although so
different in appearance from the skin, yet have the same life-processes.
Occasionally the shell moults or peels, the outer part coming off in great
flakes. Each piece grows by the addition of rings of horn at the joints,
and (like the rings of a tree) the age of turtles, except of very old
ones, can be estimated by the number of circles of horn on each piece. The
rings are very distinct in species which live in temperate climates. Here
they are compelled to hibernate during the winter, and this cessation of
growth marks the intervals between each ring. In tropical turtles the
rings are either absent or indistinct. It is to this mode of growth that
the spreading of the initials which are cut into the shell is due, just as
letters carved on the tru