) and partly
of the teeth and defensive spines of Cestracionts, such as the
living Port-Jackson Shark. The pointed and sharp-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.
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