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 neck (fig. 176), and probably possessing a simply smooth or wrinkled skin, since no traces of scales or bony integumentary plates have ever been discovered.  The tail was long, and was probably furnished at its extremity with a powerful expansion of the skin, constituting a tail-fin similar to that possessed by the Whales.  The limbs are also like those of Whales in the essentials of their structure, and in their being adapted to act as swimming-paddles.  Unlike the Whales, however, the Ichthyosaurs possessed the hind-limbs as well as the fore-limbs, both pairs having the bones flattened out and the fingers completely enclosed in the skin, the arm and leg being at the same time greatly shortened.  The limbs are thus converted into efficient “flippers,” adapting the animal for an active existence in the sea.  The different joints of the backbone (vertebrae) also show the same adaptation to an aquatic mode of life, being hollowed out at both ends, like the biconcave vertebrae of Fishes.  The spinal column in this way was endowed with the flexibility necessary for an animal intended to pass the greater part of its time in water.  Though the Ichthyosaurs are undoubtedly marine animals, there is, however, reason to believe that they occasionally came on shore,