[Illustration:  Fig. 146.—­a, Dental plate of Ceratodus serratus, Keuper; b, Dental plate of Ceratodus altus, Keuper; (After Agassiz.)]

[Illustration:  Fig 147.—­Ceratodus Fosteri, the Australian Mud-fish, reduced in size.]

Amongst the Vertebrate Animals of the Trias, the Fishes are represented by numerous forms belonging to the Ganoids and the Placoids.  The Ganoids of the period are still all provided with unsymmetrical ("heterocercal”) tails, and belong principally to such genera as Paloeoniscus and Catopterus.  The remains of Placoids are in the form of teeth and spines, the two principal genera being the two important Secondary groups Acrodus and Hybodus.  Very nearly at the summit of the Trias in England, in the Rhaetic series, is a singular stratum, which is well known as the “bone-bed,” from the number of fish-remains which it contains.  More interesting, however, than the above, are the curious palate-teeth of the Trias, upon which Agassiz founded the genus Ceratodus.  The teeth of Ceratodus (fig. 146) are singular flattened plates, composed 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 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 (Guenther); 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.