re of the pelvis and of the hind limb that some
of these ancient reptiles present the most remarkable approximation to
birds, and clearly indicate the way by which the most specialised and
characteristic features of the bird may have been evolved from the
corresponding parts of the reptile.

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 difference in form, relative size, and
direction of the corresponding bones in the two cases are very great.

But the most striking contrast between the two lies in the bones of the
leg and of that part of the foot termed the tarsus, which follows upon
the leg. In the crocodile, the fibula (_F_) is relatively large and its
lower end is complete. The tibia (_T_) has no marked crest at its upper
end, and its lower end is narrow and not pulley-shaped. There are
two rows of separate tarsal bones (_As., Ca., &c._) and four distinct
metatarsal bones, with a rudiment of a fifth.

In the bird, the fibula is small and its lower end diminishes to a
point. The tibia has a strong crest at its upper end and its lower
extremity passes into a broad pulley. There seem at first to be no
tarsal bones; and only one bone, divided at the end into three heads
for the three toes which are attached t