One branch of
these Theromorph reptiles approach the mammals so closely in the
formation of the teeth that they have received the name "of the
Theriodonts", or "beast-toothed" reptiles. Their teeth are, like
those of the mammals, divided into incisors, canines (sometimes
several inches long), and molars; and the molars have in some
cases developed cusps or tubercles. As the earlier remains of
mammals which we find are generally teeth and jaws, the
resemblance of the two groups leads to some confusion in
classifying them, but from our point of view it is not unwelcome.
It narrows the supposed gulf between the reptile and the mammal,
and suggests very forcibly the particular branch of the reptiles
to which we may look for the ancestry of the mammals. We cannot
say that these Theriodont reptiles were the ancestors of the
mammals. But we may conclude with some confidence that they bring
us near to the point of origin, and probably had at least a
common ancestor with the mammals.
The distribution of the Theriodonts suggests a further idea of
interest in regard to the origin of the mammals. It would be
improper to press this view in the present state of our
knowledge, yet it offers a plausible theory of the origin of the
mammals.
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