214 HISTORICAL PALEONTOLOGY. 



exhibit the Palaeozoic Athyris, Retzia, and Cyrtina, with the 

 Triassic Koninckia and the modern Thecidium. Finally, it is 

 here that the ancient genera Orthoceras, Cyrtoceras, and Gonia- 

 tites make their last appearance upon the scene of life, the 

 place of the last of these being taken by the more complex 

 and almost exclusively Triassic Ceratites ; whilst the still more 

 complex genus Ammonites first appears here in force, and is 

 never again wanting till we reach the close of the Mesozoic 

 period. The first representatives of the great Secondary 

 family of the Belemnitcs are also recorded from this horizon. 



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 prin- 

 cipally to such genera as Palceoniscus 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, 



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

 Ceratodus altus, Keuper. (After Agassiz.) 



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, how- 

 ever, there has been discovered in the rivers of Queensland 

 (Australia) a living species of Ceratodus (C. Fosteri, fig. 147), 



