saurs is like
that of Bats, namely, in consisting of a thin leathery expansion of
the skin which is attached to the sides of the body, and stretches
between the fore and hind limbs, being mainly supported by an
enormous elongation of certain of the digits of the hand. In
the Bats, it is the four outer fingers which are thus lengthened
out; but in the Pterosaurs, the wing-membrane is borne by a single
immensely-extended finger (fig. 178). No trace of the actual
wing-membrane itself has, of course, been found fossilised; but
we could determine that the "Pterodactyles" possessed the power
of flight, quite apart from the extraordinary conformation of
the hand. The proofs of this are to be found partly in the fact
that the breast-bone was furnished with an elevated ridge or
keel, serving for the attachment of the great muscles of flight,
and still more in the fact that the bones were hollow and were
filled with air--a peculiarity wholly confined amongst living
animals to Birds only. The skull of the Pterosaurs is long, light,
and singularly bird-like in appearance--a resemblance which is
further increased by the comparative length of the neck and the
size of the vertebrae of this region (fig. 178). The jaws, however,
unlike those of any existing Bird, were, with one exception to be
noticed hereafter, furnished with conical teeth sunk in distinct
sockets; and there was always a longer or shorter tail composed
of distinct vertebrae; whereas in all existing Birds the tail
is abbreviated, and the terminal vertebrae are amalgamated to
form a single bone, which generally supports the great feathers
of the tail.

Modern naturalists have been pretty generally agreed that the
_Pterosaurs_ should be regarded as a peculiar group of the Reptiles;
though they have been and are still regarded by high authorities,
like Professor Seeley, as being really referable to the Birds, or
as forming a class by themselves. The chief points which separate
them from Birds, as a class, are the character of the apparatus
of flight, the entirely different structure of the fore-limb, the
absence of feathers, the composition of the tail out of distinct
vertebrae, and the general presence of conical teeth sunk in distinct
sockets in the jaws. The gap between the Pterosaurs and the Birds
has, however, been greatly lessened of late by the discovery
of fossil animals (_Ichthyornis_ and _Hesperornis_) with the
skeleton proper to Birds combined with the presen