xhibit 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 _Goniatites_ 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.

[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 b