ecome 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.

[Illustration: Fig. 148.--Footprints of a Labyrinthodont
(_Cheirotherium_), from the Triassic Sandstones of Hessberg, near
Hildburghausen, Germany, reduced one-eighth. The lower figure
shows a slab, with several prints, and traversed by reticulated
sun-cracks: the upper figure shows the impression of one of the
hind-feet, one-half of the natural size. (After Sickler.)]

[Illustration: Fig. 149.--Section of the tooth of _Labryinthodon
(Mastodonsaurus) Joegeri_, showing the microscopic structure.
Greatly enlarged. Trias.]

[Illustration: Fig. 150.--a, Skull of _Labyrinthodon Joegeri_,
much reduced in size; b, Tooth of the same. Trias Wuerttemberg.]

The _Amphibians_ of the Trias all belong to the old order of
the _Labyrinthodonts_, and some of them are remarkable for their
gigantic dimensions. They were first known by their footprints,
which were found to occur plentifully in the Triassic sandstones
of Britain and the continent of Europe, and which consisted of
a double series of alternately-placed pairs of hand-shaped
impressions, the hinder print of each pair being much larger
than the one in front (fig. 148). So