THE CARBONIFEROUS PERIOD. 189 
In the Devonian rocks we meet with no other remains of 
Vertebrated animals save fishes only; but the Carboniferous 
deposits have yielded re- 
mains of the higher group 
of the Amphibians. ‘This 
class, comprising our ex- 
isting Frogs, Toads, and 
Newts, stands to some ex- 
tent in a position midway 
between the class of the 
fishes and that of the true 
reptiles, being distinguished 
from the latter by the fact Fig. 131.—Teeth of Cochliodus contortus. 
that its members invariably Carboniferous Limestone, Britain. 
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 Frotews and Alenobranchus, retain their gills 
even after acquiring their lungs, and are thus fitted indiffer- 
ently 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; dzos, life). The Amphibians also differ 
amongst themselves according as to whether they keep per- 
manently 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 integu- 
ment. 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 sup- 
ported 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 Labyrinthodonts. In the marine strata which form 
the base of the Carboniferous series these creatures have only 
been recognised by their curious hand-shaped footprints, similar 
