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.

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,