rms through which that transition was actually effected.

We are not in a position to say that the known _Ornithoscelida_ are
intermediate in the order of their appearance on the earth between
reptiles and birds. All that can be said is that, if independent
evidence of the actual occurrence of evolution is producible, then these
intercalary forms remove every difficulty in the way of understanding
what the actual steps of the process, in the case of birds, may have
been.

That intercalary forms should have existed in ancient times is a
necessary consequence of the truth of the hypothesis of evolution; and,
hence, the evidence I have laid before you in proof of the existence of
such forms, is, so far as it goes, in favour of that hypothesis.

There is another series of extinct reptiles which may be said to be
intercalary between reptiles and birds, in so far as they combine some
of the characters of both these groups; 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._

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 characterises 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.

Fig. 8.--Pterodactylus Spectabilis (Von Meyer).

There are four fingers. These four fingers a