* These Synapsid reptiles are more commonly known as Pareiasauria
or Theromorpha.
The purified air of the Permian world favoured the reptiles'
being lung-breathers, but the cold would check their expansion
for a time. The reptile, it is important to remember' usually
leaves its eggs to be hatched by the natural warmth of the
ground. But as the cold of the Permian yielded to a genial
climate and rich vegetation in the course of the Triassic, the
reptiles entered upon their memorable development. The amphibia
were now definitely ousted from their position of dominance. The
increase of the waters had at first favoured them, and we find
more than twenty genera, and some very large individuals, of the
amphibia in the Triassic. One of them, the Mastodonsaurus, had a
head three feet long and two feet wide. But the spread of the
reptiles checked them, and they shrank rapidly into the poor and
defenceless tribe which we find them in nature to-day.
To follow the prolific expansion of the reptiles in the
semi-tropical conditions of the Jurassic age is a task that even
the highest authorities approach with great diffidence. Science
is not yet wholly agreed in the classification of the vast
numbers of remains which the Mesozoic rocks have yielded, and the
affinities of the various groups are very uncertain.
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