The genera _Ctenacanthus, Gyracanthus,
Homacanthus_, &c., have been founded for the reception of these
defensive weapons, some of which indicate fishes of great size
and predaceous habits.
[Illustration: Fig. 131.--Teeth of _Cochliodus contortus_.
Carboniferous Limestone, Britain.]
[Illustration: Fig. 132.--a, Upper surface of the skull of
_Anthracosaurus Russelli_, one-sixth of the natural size: b,
Part of one of the teeth cut across, and highly magnified to
show the characteristic labyrinthine structure; c, One of the
integumentary shields or scales, one-half of the natural size.
Coal-measures, Northumberland. (After Atthey.)]
In the Devonian rocks we meet with no other remains of
Vertebrated animals save fishes only; but the Carboniferous
deposits have yielded remains of the higher group
of the _Amphibians_. This class, comprising our existing
Frogs, Toads, and Newts, stands to some extent in a position midway
between the class of the fishes and that of the true
reptiles, being distinguished from the latter by the fact
that its members invariably possess gills in their early
condition, if not throughout life; whilst they are separated from
the former by always possessing true lungs when adult, and
by the fact that the limbs (when present at all) are never in
the form of fins. The Amphibians, therefore, are all
water-breathers when young, and have respiratory organs adapted
for an aquatic mode of life; whereas, when grown up, they
develop lungs, and with these the capacity for breathing air
directly.
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