excepting that the .lines are not interrupted by any:,undulating or zigzag
inequalities. The inferior portion: of’ this-effectuM armour,! consists : of the
sternum; and the,two portions are more or less.intimately,connected, in the
various' families, by, intervening bone* which must be considered as- analogous
to the cartilages of the- ribs in the Mamrniferk,, and which, in fact; income
genera, remain, to some extent, permanently cartilaginous.
The varieties of form and;of development which this part:assumes] .in the
different groups of tortoises, constitute some of the most-constant and essential
characters by which they are: distinguished, and at the same time perfectly
accord with the structure of the neck, feet, and tail;
The Testcd! -™ or Land Tortoises, which are:exposed to continual attacks
and dangers, from which their: sluggish movements prevent, their escape,
unprovided as they are with any kind of weapons of offence, require defensive’
armour of the most secure kind, and capable of affording the strongest resisti
ance to external injury. They are.therefore provided with a hard, solid and
compact shell, forming an elevated arch, which is calculated to sustain immense,
pressure. In addition to this, the nebk’ and head ean bddrkwn wholly within
the shell ; and the: legs-the solid and clumsy structure o f winch can only
permit that slow progress by whieh these.animals are .characterized, .^are
admirably adapted, by this very peculiarity of form, .as well as by the manner
in which they are articulated, and the hard strong scales which, imhiosfeafe*
protect the anterior portion of. them, to defend the more important and vital’
parts within. Thus, when the animal is at rest, the head arid neck are wholly,
concealed, and the legs entirely occupy the anterior opening between the
upper and lower parts of the .shell, which opening, in this family, is small and
contracted, consistently with the circumscribed extent of motion required by.
the feet
I n the family o f the E m y m m , or those fresh-water tortoises which approach
most ne a rly in th e ir structure to the foregoing group, th e shell is much flatter,,
and both th e anterior and posterior openings between the dorsal; and pectoral
shields are much more expanded, in order to allow o f th e more f e e and ex,
tensive motion of the limbs in swimming. The feet also a re more slender and
moveable; and the toes, instead o f being contracted, as in the:former case, into
HISTORY A N D HABIT S OF THÈ TESTUDINATA. XIX
a solid obtuse stump, are considerably expanded, and connected by a membrane,
so as to form powerful paddles.'; The claws, too, which in the Testu-
dinidas are short and thick, are, in these, long, sharp and strong, to enable
them to tear the animal prey on which they subsist.
. In this family there is a small group in which the characters indicate a manifest
approximation to the Testudinidae. These are the Box Tortoises, in which
we find the limbs more robust, thè shell higher and less expanded, and, in short,
all the characters intermediate between the terrestrial and the fluviatile forms:
ThevSternum in. these, is divided into two or three’ portions, by one or two
transverse sections; of these the anterior at least, arid in many the anterior
and posterior, are moveable ; being connected by means of a ligamentous
or cartilaginous hinge; and are capable of more or less accurately closing the
shell, iwhen the animal is at rests :: Of these the, Terrapene clausa may be considered
as approaching most nearly tothè Land: Tortoises, and Terrapene euro-
pcea as the most closely allied to the, fresh-water group.
The T e i o n y c h i d ì e exhibit perhaps the most remarkable and interesting
structure in the: whole order.: They consist of those tortoises, inhabitants of
the fresh water, which have, no horny plates covering the shell, or any other
part of the body.. Instead of this:more usual covering, they are invested with
a strong coriaceous skin, which covers, equally the dorsum and sternum, to the
bones: of which it is closely attached : there'is’also a free margin of the. same
leathery substance:; and this circumstance,: with the remarkable flatness and
expansion of the animal, constitutes a form admirably adapted for concealment
in the, mud, at the: bottoms, of rivers and lakes ; whilst the extraordinary length
of the. neck, and the .powerful muscles by which.it ean be suddenly protruded,
enable the: animal ,to seize its prey by an instantaneous effort, without the
necessity of always quitting its retreat. :
Another peculiarity ofithisr group is, that the three outer toes, only, of each
foot, are.furnished, with claws:, The bones of the. sternum are separate; and
the fibs are.generally united only through a part of their, length.
The structure of the marine family, theLHEnoN.iABJE,(exhibits the natatorial
character in the highest degree, : and rat-once points, out these animals as permanent
inhabitants of the sea. ,’ The whole bodycis fla t,th e shell so small as