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.

[Illustration: FIG. 6.--BIRD. ORNITHOSCELIDAN. CROCODILE.

(The letters have the same signification in all the figures. Il.,
Ilium; a, anterior end; b, posterior end; Is., ischium; Pb.,
pubis; T, tibia; F, fibula; As., astragalus; Ca., calcaneum; 1,
distal portion of the tarsus; i., ii., 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