their apparent total absence in Palaeozoic strata, is a strong presumption in favour of the view that the order of the Cuttle-fishes did not come into existence till the commencement of the Mesozoic period.  The great majority of the skeletons of this kind which are found in the Jurassic rocks belong to the great extinct family of the “Belemnites” (Belemnitidoa), which, so far as known, is entirely confined to rocks of Secondary age.  From its pointed, generally cylindro-conical form, the skeleton of the Belemnite is popularly known as a “thunderbolt”. (fig. 173, C).  In its perfect condition—­in which it is, however, rarely obtainable—­the skeleton consists of a chambered conical shell (the “phragmacone"), the partitions between the chambers of which are pierced by a marginal tube or “siphuncle.”  This conical shell—­curiously similar in its structure to the external shell of the Nautilus—­is extended 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