Fig. 131.—Teeth of Cochliodus contortus. Carboniferous Limestone, Britain. 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 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 in character to those which occur in the Triassic rocks, and which will be subsequently spoken of under the name of Cheirotherium. In the Coal-measures of Britain, the continent of Europe, and North America, however, many bones of these animals have been found, and we are now tolerably well acquainted with a considerable number of forms. All of them seem to have belonged to the division of Amphibians in which the long tail of the young is permanently retained; and there is evidence that some of them kept the gills also throughout life. The skull is of the characteristic Amphibian type (fig. 132, a), with two occipital condyles, and having its surface
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.) singularly pitted and sculptured; and the vertebræ are hollowed out at both ends. The lower surface of the body was defended by an armour of singular integumentary shields or scales (fig. 132, c); and an extremely characteristic feature (from which the entire group derives its name) is, that the walls of the teeth are deeply folded, so as to give rise to an extraordinary "labyrinthine" pattern when they are cut across (fig. 132, b). Many of the Carboniferous Labyrinthodonts are of no great size, some of them very small, but others attain comparatively gigantic dimensions, though all fall short in this respect of the huge examples of this group which occur in the Trias. One of the largest, and at the same time most characteristic, forms of the Carboniferous series, is the genus Anthracosaurus, the skull of which is here figured.
No remains of true Reptiles, Birds, or Quadrupeds have as yet been certainly detected in the Carboniferous deposits in any part of the world. It should, however, be mentioned, that Professor Marsh, one of the highest authorities on the subject, has described from the Coal-formation of Nova Scotia certain vertebræ which he believes to have belonged to a marine reptile (Eosaurus Acadianus), allied to the great Ichthyosauri of the Lias. Up to this time no confirmation of this determination has been obtained by the discovery of other and more unquestionable remains, and it therefore remains doubtful whether these bones of Eosaurus may not really belong to large Labyrinthodonts.
LITERATURE.
The following list contains some of the more important of the original sources of information to which the student of Carboniferous rocks and fossils may refer:—
Also numerous memoirs by Huxley, Davidson, Martin Duncan, Professor Young, John Young, R. Etheridge, jun., Baily, Carruthers, Dawson, Binney, Williamson, Hooker, Jukes, Geikie, Rupert Jones, Salter, and many other British and foreign observers.
CHAPTER XIV.
THE PERMIAN PERIOD.