220 HISTORICAL PALEONTOLOGY. 



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 prin- 

 cipally to such genera as Palaoniscus 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 bones beneath, covered superficially with 

 a layer of enamel. Each plate is approximately triangular, 

 one margin (which we 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 recognized that these 

 singular bodies were undoubtedly the teeth of fishes, we were 

 entirely ignorant as to their precise relation to the animal, or 



Fig. 146. a, Dental plate of Ceratodus serratus, Keuper ; b, Dental plate of 

 Ceratodus altus, Keuper. (After Agassiz.) 



as to the exact affinities of the fish thus armed. Lately, how- 

 ever, 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 



