: Fig. 173.--A, Restoration of the animal of the
Belemnite; B, Diagram showing the complete skeleton of a Belemnite,
consisting of the chambered phragmacone (a), the guard (b), and
the horny pen (c); C, Specimen of _Belemnites canaliculatus_,
from the Inferior Oolite. (After Phillips.)]

[Illustration: Fig. 174.--_Tetragonolepis (restored), and scales
of the same. Lias.]

Coming next to the _Vertebrates_, we find that the Jurassic _Fishes_
are still represented by _Ganoids_ and _Placoids_. The Ganoids,
however, unlike the old forms, now for the most part possess
nearly or quite symmetrical ("homocercal") tails. A characteristic
genus is _Tetragonolepis_ (fig. 174), with its deep compressed
body, its rhomboidal, closely-fitting scales, and its single
long dorsal fin. Amongst the _Placoids_ the teeth of true Sharks
(_Notidanus_) occur for the first time; but by far the greater
number of remains referable to this group are still the fin-spines
and teeth of "Cestracionts," resembling the living Port-Jackson
Shark. Some of these teeth are pointed (_Hybodus_); but others
are rounded, and are adapted for crushing shell-fish. Of these
latter, the commonest are the teeth of _Acrodus_ (fig. 175), of
which the hinder ones are of an elongated form, with a rounded
surface, covered with fine transverse striae proceeding from a
central longitudinal line. From their general form and striation,
and their dark colour, these teeth are commonly called "fossil
leeches" by the quarrymen.

[Illustration: Fig. 175.--Tooth of _Acrodus nobilis_. Lias.]

The Amphibian group of the _Labyrinthodonts_, which was so
extensively developed in the Trias, appears to have become extinct,
no representative of the order having hitherto been detected in
rocks of Jurassic age.

[Illustration: Fig. 176.--_Ichthyosaurus communis. Lias.]

Much more important than the Fishes of the Jurassic series are
the _Reptiles_, which are both very numerous, and belong to a
great variety of types, some of these being very extraordinary
in their anatomical structure. The predominant group is that
of the "Enaliosaurs" or "Sea-lizards," divided into two great
orders, represented respectively by the _Ichthyosaurus_ and the
_Plesiosaurus_.

The _Ichthyosauri_ or "Fish-Lizards" are exclusively Mesozoic
in their distribution, ranging from the Lias to the Chalk, but
abounding especially in the former. They were huge Reptiles, of
a fish-like form, with a hardly conspicuous