and, which, as they possessed
the power of flight, may seem, at first sight, to be nearer
representatives of the forms by which the transition from the reptile to
the bird was effected, than the _Ornithoscelida_.

[Illustration: FIG. 8.--PTERODACTYLUS SPECTABILIS (Von Meyer).]

These are the _Pterosauria_, or Pterodactyles, the remains of which are
met with throughout the series of Mesozoic rocks, from the lias to the
chalk, and some of which attained a great size, their wings having a
span of eighteen or twenty feet. These animals, in the form and
proportions of the head and neck relatively to the body, and in the fact
that the ends of the jaws were often, if not always, more or less
extensively ensheathed in horny beaks, remind us of birds. Moreover,
their bones contained air cavities, rendering them specifically lighter,
as is the case in most birds. The breast-bone was large and keeled, as in
most birds and in bats, and the shoulder girdle is strikingly similar to
that of ordinary birds. But, it seems to me, that the special
resemblance of pterodactyles to birds ends here, unless I may add the
entire absence of teeth which characterizes the great pterodactyles
(_Pteranodon_), discovered by Professor Marsh. All other known
pterodactyles have teeth lodged in sockets. In the vertebral column and
the hind limbs there are no special resemblances to birds, and when we
turn to the wings they are found to be constructed on a totally
different principle from those of birds.

There are four fingers. These four fingers are large, and three of them,
those which answer to the thumb and two following fingers in my
hand--are terminated by claws, while the fourth is enormously prolonged
and converted into a great jointed style. You see at once, from what I
have stated about a bird's wing, that there could be nothing less like a
bird's wing than this is. It concluded by general reasoning that this
finger had the office of supporting a web which extended between it and
the body. An existing specimen proves that such was really the case, and
that the pterodactyles were devoid of feathers, but that the fingers
supported a vast web like that of a bat's wing; in fact, there can be no
doubt that this ancient reptile flew after the fashion of a bat.

Thus though the pterodactyle is a reptile which has become modified in
such a manner as to enable it to fly, and therefore, as might be
expected, presents some points of resemblance to othe