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.

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 to it, appears in the place of the
metatarsus.

In a 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.

[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.,