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.
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