forwards into a horny "pen," and is sunk in a corresponding conical pit (fig. 173, B), excavated in the substance of a nearly cylindrical fibrous body or "guard," which projects backwards for a longer or shorter distance, and is the part most usually found in a fossil condition. Many different kinds of _Belemnites_ are known, and their guards literally swarm in many parts of the Jurassic series, whilst some specimens attain very considerable dimensions. Not only is the internal skeleton known, but specimens of _Belemnites_ and the nearly allied _Belemnoteuthis_ have been found in some of the fine-grained sediments of the Jurassic formation, from which much has been learnt even as to the anatomy of the soft parts of the animal. Thus we know that the Belemnites were in many respects comparable with the existing Calamaries or Squids, the body being furnished with lateral fins, and the head carrying a circle of ten "arms," two of which were longer than the others (fig. 173, A). The suckers on the arms were provided, further, with horny hooks; there was a large ink-sac; and the mouth was armed with horny mandibles resembling in shape the beak of a parrot. [Illustration: 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 striæ 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.