Tortoises (Testudinata, or Chelonians) belong to a very numerous order
of reptiles, the usual form of which is too well known to require
description. They are shut up, as it were, in a box and breast-plate:
the carapace and plastron, in reality, are external developments of
certain parts of the skeleton.
The land tortoises have the strongest plastrons. In some species it is
slightly movable, but generally fixed by a uniting suture. In one--the
pyxis--the plastron is furnished with a transverse hinge, so that the
animal can retract its head and fore-limbs within the carapace, and
close the plastron upon it, first shutting them in. In another--the
kinixis--the carapace has the posterior portion distinct from the
anterior, and movable, so as to shield the hind-limbs and tail.
In water tortoises, or turtles, as they are generally called, the
plastron is united to the edges of the carapace by intervening
cartilage, and not by suture. The jaws of tortoises are not furnished
with teeth, but are cased in horny coverings, resembling somewhat the
sharp hooked beak of a parrot; which enable them either to crop and
mince the vegetable aliment on which most of them live, or to masticate
the small, living animals, such as birds and reptiles, of which the food
of others consists. Round the outside of this beak are thick fleshy
lips.
In the curious matamata, the jaws of which open very wide, these parts,
instead of being armed by a strong beak, are protected by a sheath of
horn.
In the land tortoises, the feet are stump-like, the toes being enveloped
in the skin, so that they can move but slowly. The marsh and lake
tortoises have their feet palmated, to enable them to move either on the
water or on land. In the turtles, these limbs appear in the form of
broad, flat, undivided paddles, well-adapted for moving in the water,
but awkward as instruments of locomotion, even on the level, sandy
shores to which they resort at the breeding season.
The tortoise has a fleshy tongue like that of a parrot. The brain is
but slightly developed, scarcely filling the cavity of the skull in the
marine species. At the same time, the animal possesses great muscular
irritability, and extreme tenacity of life. All are oviparous, and bury
their eggs, which are hatched by the warmth of the sun. The water
tortoises, when seen below the surface, move like birds in the air, the
paddles flapping like wings.
The order is divided into four groups: first, Chersians, or the land
tortoises; second, the Elodians, or marsh tortoises; third, the
Potamians, or river tortoises; fourth, the Thalassians, or sea
tortoises, generally called turtles. These groups are again variously
subdivided.
The waters of Tropical America abound with the second and third
families. The Elodians, found in the shallow pools of the Amazonian
Valley, swim with facility, and move quickly over the ground. They feed
not only on vegetables, but prey on living animals--river molluscs, and
other water creatures.
The Potamians, which are found in vast numbers in the larger rivers,
grow to a great size,--some weighing seventy pounds. They feed much as
the last described. They swim with ease, both on the surface and at
mid-water. The upper part of the body is generally brown or grey, with
regular dotted spots; while the under parts are pale white, rosy, and
bluish. When they seize their food, they dart out their heads and long
necks with the rapidity of arrows, and bite sharply with their trenchant
beaks, not letting go till they have taken the piece out. The females
are said to be far more numerous than the males; indeed, Father Gumilla,
describing the turtles of the Orinoco, states what might be doubted,--
that "in each nest of eggs there is one, larger than the rest, from
which the male is hatched. All the others are females." The eggs are
spherical; their shell solid, but membraneous or slightly calcareous.
A further description of them will be given when the mode in which they
are captured is described. The species, however, deserves particular
notice.