ig. 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. Some of them, like the Frogs and Newts, lose their
gills altogether on attaining the adult condition; but others,
such as the living _Proteus_ and _Menobranchus_, retain
their gills even after acquiring their lungs, and are thus fitted
indifferently for an aquatic or terrestrial existence. The name of
"Amphibia," though applied to the whole class, is thus not
precisely appropriate except to these last-mentioned forms
(Gr. _amphi_, both; _bios_, life). The Amphibians also
differ amongst themselves according as to whether they keep
permanently the long tail which they all possess when young (as
do the Newts and Salamanders), or lose this appendage when
grown up (as do the Frogs and Toads). Most of them have
naked skins, but a few living and many extinct forms have
hard structures in the shape of scales developed in the integument.
All of them have well-ossified skeletons, though some
fossil types are partially deficient in this respect; and all of
them which possess limbs at all have these appendages supported
by bones essentially similar to those found in the limbs
of the higher Vertebrates. All the Carboniferous Amphibians
belong to a group which has now wholly passed away--namely,
that of the _Labyrinthodon