It is more difficult
to identify Monotreme remains, but the fact that Monotremes have
survived to this day in Australia, and the resemblance of some of
the Mesozoic teeth to those found for a time in the young
Duckbill justify us in assuming that a part of the Mesozoic
mammals correspond to the modern Monotremes. Not single specimen
of any higher, or placental, mammal has yet been found in the
whole Mesozoic Era.
We must, however, beware of simply transferring to the Mesozoic
world the kinds of Monotremes and Marsupials which we know in
nature to-day. In some of the excellent "restorations" of
Mesozoic life which are found in recent illustrated literature
the early mammal is represented with an external appearance like
that of the Duckbill. This is an error, as the Duckbill has been
greatly modified in its extremities and mouth-parts by its
aquatic and burrowing habits. As we have no complete skeletons of
these early mammals we must abstain from picturing their external
appearance. It is enough that the living Monotreme and Marsupial
so finely illustrate the transition from a reptilian to a
mammalian form. There may have been types more primitive than the
Duckbill, and others between the Duckbill and the Marsupial.
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