iii., iv.; metatarsal bones.)]

On comparing the pelvis and hind limb of the ornithoscelidan with that
of the crocodile, on the one side, and that of the bird, on the other
(Fig. 6), it is obvious that it represents a middle term between the
two. The pelvic bones approach the form of those of the birds, and the
direction of the pubis and ischium is nearly that which is
characteristic of birds; the thigh bone, from the direction of its head,
must have lain close to the body; the tibia has a great crest; and,
immovably fitted on to its lower end, there is a pulley-shaped bone,
like that of the bird, but remaining distinct. The lower end of the
fibula is much more slender, proportionally, than in the crocodile. The
metatarsal bones have such a form that they fit together immovably,
though they do not enter into bony union; the third toe is, as in the
bird, longest and strongest. In fact, the ornithoscelidan limb is
comparable to that of an unhatched chick.

Taking all these facts together, it is obvious that the view, which was
entertained by Mantell and the probability of which was demonstrated by
your own distinguished anatomist, Leidy, while much additional evidence
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 conclusi