There also occur not uncommonly the flattened teeth of Rays (fig.
225), consisting of flat bony pieces placed close together, and
forming "a kind of mosaic pavement on both the upper and lower
jaws" (Owen).
[Illustration: Fig. 223.--_Rhombus minimus_, a small fossil Turbot
from the Eocene Tertiary, Monte Bolca.]
[Illustration: Fig. 224.--Tooth of _Otodus obliquus_. Eocene.]
[Illustration: Fig. 225.--Flattened dental plates of a Ray
(_Myliobatis Edwardsii_). Eocene.]
In the class of the _Reptiles_, the disappearance of the
characteristic Mesozoic types is as marked a phenomenon as the
introduction of new forms. The Ichthyosaurs, the Plesiosaurs,
the Pterosaurs, and the Mosasaurs of the Mesozoic, find no
representatives in the Eocene Tertiary; and the same is true of the
Deinosaurs, if we except a few remains from the doubtfully-situated
"Lignitic formation" of the United States, On the other hand, all
the modern orders of Reptiles are known to have existed during
the Eocene period. The _Chelonians_ are represented by true marine
Turtles, by "Terrapins" (_Emydidoe_), and by "Soft Tortoises"
(_Trionycidoe_). The order of the Snakes and Serpents (_Ophidia_)
makes its appearance here, for the first time under several
forms--all of which, however, are referable to the non-venomous
group of the "Constricting Serpents" (_Boidoe_).
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