in the same direction has been furnished by Professor Cope, that some of
these animals may have walked upon their hind legs, as birds do,
acquires great weight. In fact, there can be no reasonable doubt that
one of the smaller forms of the _Ornithoscelida_, _Compsognathus_, the
almost entire skeleton of which has been discovered in the Solenhofen
slates, was a bipedal animal. The parts of this skeleton are somewhat
twisted out of their natural relations, but the accompanying figure
gives a just view of the general form of _Compsognathus_ and of the
proportions of its limbs; which, in some respects, are more completely
bird-like than those of other _Ornithoscelida_.

[Illustration: FIG. 7.--RESTORATION OF COMPSOGNATHUS LONGIPES.]

We have had to stretch the definition of the class of birds so as to
include birds with teeth and birds with paw-like fore-limbs and long
tails. There is no evidence that _Compsognathus_ possessed feathers;
but, if it did, it would be hard indeed to say whether it should be
called a reptilian bird or an avian reptile.

As _Compsognathus_ walked upon its hind legs, it must have made tracks
like those of birds. And as the structure of the limbs of several of the
gigantic _Ornithoscelida_, such as _Iguandon_, leads to the conclusion
that they also may have constantly, or occasionally, assumed the same
attitude, a peculiar interest attaches to the fact that, in the Wealden
strata of England, there are to be found gigantic footsteps, arranged in
order like those of the _Brontozoum_, and which there can be no
reasonable doubt were made by some of the _Ornithoscelida_, the remains
of which are found in the same rocks. And, knowing that reptiles that
walked upon their hind legs and shared many of the anatomical characters
of birds did once exist, it becomes a very important question whether