class Mammalia. They possess also other characters
that are unmistakably mammalian. Leaving out of consideration the
structure of the internal organs, they have teeth implanted in sockets
in the jaws, four limbs, and a hairy covering to the skin, so that
they possess more decidedly mammalian characters than some other
members of the class, such as the marine whales and dolphins
(_Cetacea_) and manatees (_Sirenia_), which are still often spoken of
as fishes. In point of fact, although organized for flight, the Bat
may, without any violence to language, be spoken of as a _quadruped_,
for its fore-limbs contain all the parts found in those of other
mammals fully developed, and they come into use when the creature is
walking on the ground.

Perhaps the special characteristics of the Bats will be brought out
most distinctly by a comparison of their structure with that of a
bird, seeing that the modification of the fore-limbs into wings is
their most striking distinction from other Mammalia; for, although
some other members of the class are spoken of as "flying," such as the
Flying Squirrels, Flying Lemurs, and Flying Phalangers, these
creatures do not really fly, but merely glide through the air to
considerable distances by the action of a broad fold of skin which
runs down each side of the body, and which, when stretched between
the extended limbs, buoys the creatures up in the air after the
fashion of a parachute.

Most of us must have had occasion to pick the bones of a bird's wing,
a piece of practical anatomy which may serve us in good stead at
present. They consist of a long bone, which may be called the arm-bone
(_humerus_), jointed to the shoulder-bones (the so-called "side-bones"
of a fowl or turkey), followed by a pair of parallel bones
constituting the fore-arm, at the end of which we find two or three
small bones, then two parallel bones united at their extremities, and
some smaller joints terminating the whole.

We need say nothing about the arm-bone and the two bones of the
fore-arm, the peculiarity of bird-structure lying chiefly in the
terminal portion of the limb, or the hand. Here we find, after two
little bones forming the wrist, a pair of long bones as above
described, firmly united both at base and apex, and on the outside of
the base of these, close to the wrist, a small bone, which may be
either free or soldered to the others, and which represents the thumb
in the human hand. At the other end of t