ted inwards, and supports part of the posterior
margin of an accessory membrane of flight, extending from the tail or
posterior extremity of the body to the hind-limbs, and known as the
interfemoral membrane. The penis is pendent; the testes are abdominal
or inguinal; the teats, usually two in number, thoracic; the uterus is
simple or with more or less long cornua; the placenta discoidal and
deciduate; and the smooth cerebral hemispheres do not extend backwards
over the cerebellum. The teeth comprise incisors, canines, premolars and
molars; and the dental formula never exceeds i. 2/3, c. 1/1, p. 3/8, m.
3/3; total 38. Despite the forward position of the teats, which is
merely an adaptive feature, bats are evidently mammals of low
organization, and are most nearly related to the Insectivora.

In consequence of the backward direction of the knee, a bat, when placed
on the ground, rests on all fours, having the knees directed upwards,
while the foot is rotated forwards and inwards on the ankle. Walking is
thus a kind of shuffle; but, notwithstanding a general belief, bats can
take wing from the walking posture.

The bones of the skeleton are characterized by their slenderness and the
great size of the medullary canals in those of the extremities. The
vertebral column is short, and the vertebrae differ but slightly in
number and form throughout the group. The general number of dorso-lumbar
vertebrae is 17, whereof 12 are dorsal; the cervical vertebrae are
broad, but short. Except in fruit-bats (_Pteropodidae_), the vertebrae,
from the third cervical backwards, are devoid of spinous processes. From
the first dorsal to the last lumbar the vertebral column forms a single
curve, most pronounced in the lumbar region. The bodies of the vertebrae
are but slightly movable on each other, and in old individuals become
partially welded. The caudal vertebrae are cylindrical bones without
processes; their number and length varying in allied species. The
development of these vertebrae is correlated with habits, the long tail
in the insectivorous species supporting and controlling the position of
the interfemoral membrane which aids bats in their doubling motions when
in pursuit of insects by acting as a rudder, and assists them in the
capture of the larger insects. In the fruit-bats this is not required,
and the tail is rudimentary or absent. In all bats the presternum has a
prominent keel for the attachment of the great pectoral muscles