Fig. 146.—a, Dental plate of Ceratodus serratus, Keuper; b, Dental plate of Ceratodus altus, Keuper; (After Agassiz.) of spongy bone beneath, covered superficially with a layer of enamel. Each plate is approximately triangular, one margin (which we now know to be the outer one) being prolonged into prongs or conical prominences, whilst the surface is more or less regularly undulated. Until recently, though the master-mind of Agassiz recognised that these singular bodies were undoubtedly the teeth of fishes, we were entirely ignorant as to their precise relation to the animal, or as to the exact affinities of the fish thus armed. Lately, however, there has been discovered in the rivers of Queensland (Australia) a living species of Ceratodus (C. Fosteri, fig. 147), with teeth precisely similar to those of its Triassic predecessor; and we thus have become acquainted with the use of these structures and the manner in which they
Fig. 147.—Ceratodus Fosteri, the Australian Mud-fish, reduced in size. were implanted in the mouth. The palate carries two of these plates, with their longer straight sides turned towards each other, their sharply-sinuated sides turned outwards, and their short straight sides or bases directed backwards. Two similar plates in the lower jaw correspond to the upper, their undulated surfaces fitting exactly to those of the opposite teeth. There are also two sharp-edged front teeth, which are placed in the front of the mouth in the upper jaw; but these have not been recognised in the fossil specimens. The living Ceratodus feeds on vegetable matters, which are taken up or tom off from plants by the sharp front teeth, and then partially crushed between the undulated surfaces of the back teeth (Günther); and there need be little doubt but that the Triassic Ceratodi followed a similar mode of existence. From the study of the living Ceratodus, it is certain that the genus belongs to the same group as the existing Mud-fishes (Dipnoi); and we therefore learn that this, the highest, group of the entire class of Fishes existed in Triassic times under forms little or not at all different from species now alive; whilst it has become probable that the order can be traced back into the Devonian period.
The Amphibians of the Trias all belong to the old order of the Labyrinthodonts, and some of them are remarkable for their gigantic dimensions. They were first known by their footprints, which were found to occur plentifully in the Triassic sandstones of Britain and the continent of Europe, and which consisted of a double series of alternately-placed pairs of hand-shaped impressions, the hinder print of each pair being much larger than the one in front (fig. 148). So like were these impressions to the shape of the human hand, that the at that time unknown animal which produced them was at once christened Cheirotherium, or "Hand-beast." Further discoveries, however, soon showed that the footprints of Cheirotherium were really produced by species of Amphibians which, like the existing Frogs, possessed hind-feet of a much larger size than the fore-feet,
Fig. 148.—Footprints of a Labyrinthodont (Cheirotherium), from the Triassic Sandstones of Hessberg, near Hildburghausen, Germany, reduced one-eighth. The lower figure shows a slab, with several prints, and traversed by reticulated sun-cracks: the upper figure shows the impression of one of the hind-feet, one-half of the natural size. (After Sickler.) and to which the name of Labyrinthodonts was applied in consequence of the complex microscopic structure of the teeth (fig. 149). In the essential details of their structure, the Triassic Labyrinthodonts did not differ materially from their predecessors in the Coal-measures and Permian rocks. They possessed the same frog-like skulls (fig. 150), with a lizard-like body, a long tail, and comparatively feeble limbs. The hind-limbs were stronger and longer than the fore-limbs, and the lower surface of the body was protected by an armour of bony plates. Some of the Triassic Labyrinthodonts must have attained dimensions utterly unapproached amongst existing Amphibians, the skull of Labyrinthodon Jœgeri (fig. 150) being upwards
Fig. 149.—Section of the tooth of Labryinthodon (Mastodonsaurus) Jœgeri, showing the microscopic structure. Greatly enlarged. Trias.