and Cochliodus (fig. 131) may be mentioned as specially characteristic.  The fin-spines are mostly similar to those so common in the Devonian deposits, consisting of hollow defensive spines implanted in front of the pectoral or other fins, usually slightly curved, often superficially ribbed or sculptured, and not uncommonly serrated or toothed.  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.  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