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.
Pages:
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367