2h4 HISTORICAL PALAONTOLOGY. 
exhibit the Paleozoic Athyris, Retzia, and Cyrtina, with the 
Triassic Koninckia and the modern Zhecedium. Finally, it is 
here that the ancient genera Orthoceras, Cyrtoceras, and Gonta- 
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 Belemnites are also recorded from this horizon. 
Amongst the Vertebrate Animals of the Trias, the Fzshes are 
represented by numerous forms belonging to the Gazozds and 
the Pézcoids. The Ganoids of the period are still all provided 
with unsymmetrical (‘‘heterocercal”) tails, and belong prin- 
cipally to such genera as Padleoniscus and Catopterus. ‘The 
remains of Placoids are in the form of teeth and spines, the 
two principal genera being the two important Secondary 
sroups Acrodus and Hybodus. Very nearly at the summit 
of the Trias in England, in the Rhetic 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 serratus, Keuper; 4, 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. Fostert, fig. 147), 
