In Fig. 6, the pelvis and hind limbs of a crocodile, a three-toed bird,
and an ornithoscelidan are represented side by side; and, for facility
of comparison, in corresponding positions; but it must be recollected
that, while the position of the bird's limb is natural, that of the
crocodile is not so. In the bird, the thigh-bone lies close to the body,
and the metatarsal bones of the foot (ii., iii., iv., Fig. 6) are,
ordinarily, raised into a more or less vertical position; in the
crocodile, the thigh-bone stands out at an angle from the body, and the
metatarsal bones (i., ii., iii., iv., Fig. 6) lie flat on the ground.
Hence, in the crocodile, the body usually lies squat between the legs,
while, in the bird, it is raised upon the hind legs, as upon pillars.
In the crocodile, the pelvis is obviously composed of three bones on
each side: the ilium (Il.), the pubis (Pb.), and the ischium (Is.). In
the adult bird there appears to be but one bone on each side. The
examination of the pelvis of a chick, however, shows that each half is
made up of three bones, which answer to those which remain distinct
throughout life, in the crocodile. There is, therefore, a fundamental
identity of plan in the construction of the pelvis of both bird and
reptile; though the differences in form, relative size, and direction of
the corresponding bones in the two cases are very great.
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