o it, appears in the place of the
metatarsus.

In the young bird, however, the pulley-shaped apparent end of the tibia
is a distinct bone, which represents the bones marked _As., Ca.,_ in the
crocodile; while the apparently single metatarsal bone consists of three
bones, which early unite with one another and with an additional bone,
which represents the lower row of bones in the tarsus of the crocodile.

In other words, it can be shown by the study of development that the
bird's pelvis and hind limb are simply extreme modifications of the
same fundamental plan as that upon which these parts are modelled in
reptiles.

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.

Fig. 6.--Bird. Ornithoscelidan. Crocodile. The letters have
the same signification in all the figures. _Il.,_ Ilium;
_a._ anterior end; _b._ posterior end; _Ia._ ischium; _Pb.,_
pubis; _T,_ tibia; _F,_ fibula; _As.,_ astragalus; _Ca.,_
calcaneum; I, distal portion of the tarsus; i., ii., iii.,
iv., metatarsal bones.

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 i