No one
will deny that the Hipparion is intermediate between the existing horse and
certain other ungulate forms. What a wonderful connecting link in the
chain of mammals is the Typotherium from South America, as the name given
to it by Professor Gervais expresses, and which cannot be placed in any
existing order. The Sirenia form a very distinct group of the mammals, and
one of the most remarkable peculiarities in existing dugong and lamentin is
the entire absence of hind limbs, without even a rudiment being left; but
the extinct Halitherium had, according to Professor Flower, an ossified
thigh-bone "articulated to a well-defined acetabulum in the pelvis," and it
thus makes some approach to ordinary hoofed quadrupeds, to which the
Sirenia are in other respects allied. The cetaceans or whales are widely
different from all other mammals, but the tertiary Zeuglodon and Squalodon,
which have been placed by some naturalists in an order by themselves, are
considered by Professor Huxley to be undoubtedly cetaceans, "and to
constitute connecting links with the aquatic carnivora."
Even the wide interval between birds and reptiles has been shown by the
naturalist just quoted to be partially bridged over in the most unexpected
manner, on the one hand, by the ostrich and extinct Archeopteryx, and on
the other hand by the Compsognathus, one of the Dinosaurians--that group
which includes the most gigantic of all terrestrial reptiles.
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