rp-edged teeth of
true Sharks are very abundant in some beds, such as the Upper
Greensand, and are beautifully preserved. The teeth of some forms
(_Carcharias_, &c.) attain occasionally a length of three or four
inches, and indicate the existence in the Cretaceous seas of
huge predaceous fishes, probably larger than any existing Sharks.
The remains of _Cestracionts_ consist partly of the flattened
teeth of genera such as _Acrodus_ and _Ptychodus_ (the latter
confined to rocks of this age), and partly of the pointed teeth
of _Hybodus_, a genus which dates from the Trias. In this genus
the teeth (fig. 206) consist of a principal central cone, flanked
by minor lateral cones; and the fin-spines (fig. 207) are
longitudinally grooved, and carry a series of small spines on
their hinder or concave margin. Lastly, the great modern order
of the Bony Fishes or _Teleosteans_ makes its first appearance
in the Upper Cretaceous rocks, where it is represented by forms
belonging to no less than three existing groups--namely, the
Salmon family (_Salmonidoe_), the Herring family (_Clupeidoe_),
and the Perch family (_Percidoe_). All these fishes have thin,
horny, overlapping scales, symmetrical ("homocercal") tails,
and bony skeletons. The genus _Beryx_ (fig. 208, 1) is one
represented by existing species at the present day, and belongs
to the Perch family. The genus _Osmeroides_, again (fig. 208,
2), is supposed to be related to the living Smelts (_Osmerus_),
and, therefore, to belong to the Salmon tribe.

[Illustration: Fig. 205.--Guard of _Belemnitella mucronata_. White
Chalk.]

[Illustration: Fig. 206.--Tooth of _Hybodus_.]

[Illustration: Fig. 207.--Fin-spine of _Hybodus_. Lower Greensand.]

[Illustration: Fig. 208.--1, _Beryx Lewesiensis_, a Percoid fish
from the Chalk; 2, _Osmeroides Mantelli_, a Salmonoid fish from
the Chalk.]

No remains of _Amphibians_ have hitherto been detected in any part
of the Cretaceous series; but _Reptiles_ are extremely numerous,
and belong to very varied types. As regards the great extinct groups
of Reptiles which characterise the Mesozoic period as a whole, the
huge "Enaliosaurs" or "Sea-Lizards" are still represented by the
_Ichthyosaur_ and the _Plesiosaur_. Nearly allied to the latter
of these is the _Elasmosaurus_ of the American Cretaceous, which
combined the long tail of the Ichthyosaur with the long neck
of the Plesiosaur. The length of this monstrous Reptile could not
have been less th