Amongst the Jurassic Univalves (Gasteropoda) there are many examples of the ancient and long-lived Pleurotomaria; but on the whole the Univalves begin to have a modern aspect. The round-mouthed ("holostomatous"), vegetable-eating Sea-snails, such as the Limpets (Patellidœ), the Nerites (Nerita), the Turritellœ, Chemnitziœ, &c., still hold a predominant place. The two most noticeable genera of this group are Cerithium and Nerinœa—the former of these attaining great importance in the Tertiary and Recent seas, whilst the latter (fig. 169) is highly characteristic of the Jurassic series, though not exclusively confined to it. One of the
Fig. 169.—Nerinœa Goodhallii, one-fourth of the natural size. The left-hand figure shows the appearance presented by the shell when vertically divided. Coral-rag, England. limestones of the Jura, believed to be of the age of the Coral-rag (Middle Oolite) of Britain, abounds to such an extent in the turreted shells of Nerinœa as to have gained the name of "Calcaire à Nérinées." In addition to forms such as the preceding, we now for the first time meet, in any force, with the Carnivorous Univalves, in which the mouth of the shell is notched or produced into a canal, giving rise to the technical name of "siphonostomatous" applied to the shell. Some of the carnivorous forms belong to extinct types, such as the Purpuroidea of the Great Oolite; but others are referable to well-known existing genera. Thus we meet here with species of the familiar groups of the Whelks (Buccinum), the Spindle-shells (Fusus), the Spider-shells (Pteroceras), Murex, Rostellaria, and others which are not at present known to occur in any earlier formation.
Amongst the Wing-shells (Pteropoda), it is sufficient to mark the final appearance in the Lias of the ancient genus Conularia.
Lastly, the order of the Cephalopoda, in both its Tetrabranchiate and Dibranchiate sections, undergoes a vast development in the Jurassic period. The old and comparatively simple genus Nautilus is still well represented, one species being very similar to the living Pearly Nautilus (N. Pompilius); but the Orthocerata and Goniatites of the Trias have finally disappeared; and the great majority of the Tetrabranchiate forms are referable to the comprehensive genus Ammonites, with its many sub-genera and its hundreds of recorded species. The shell in Ammonites is in the form of a flat spiral, all the coils of which are in contact (figs. 170 and 171). The innermost whorls of the shell are more or less concealed; and the body-chamber is elongated and narrow, rather than expanded towards the mouth. The tube or siphuncle which runs through the air-chambers is placed on the dorsal or convex side of the shell; but the principal character which distinguishes Ammonites from Goniatites and
Fig. 170.—Ammonites Humphresianus. Inferior Oolite. Ceratites is the wonderfully complex manner in which the septa, or partitions between the air-chambers, are folded and undulated. To such an extent does this take place, that the edges of the septa, when exposed by the removal of the shell-substance,
Fig. 171.—Ammonites bifrons. Lias. present in an exaggerated manner the appearance exhibited by an elaborately-dressed shirt-frill when viewed edgewise. The species of Ammonites range from the Carboniferous to the Chalk; but they have not been found in deposits older than the Secondary, in any region except India; and they are therefore to be regarded as essentially Mesozoic fossils. Within these limits, each formation is characterised by particular species, the number of individuals being often very great, and the size which is sometimes attained being nothing short of gigantic. In the Lias, particular species of Ammonites may succeed one another regularly, each having a more or less definite horizon, which it does not transgress. It is thus possible to distinguish a certain number of zones, each characterised by a particular Ammonite, together with other associated fossils. Some of these zones are very persistent and extend over very wide areas, thus affording valuable aid to the geologist in his determination of rocks. It is to be remembered, however, that there are other species which are not thus restricted in their vertical range, even in the same formations in which definite zones occur.
The Cuttle-fishes or Dibranchiate Cephalopods constitute a feature in the life of the Jurassic period little less conspicuous and striking than that afforded by the multitudinous and varied chambered shells of the Ammonitidœ. The remains by which these animals are recognised are necessarily less perfect, as a rule, than those of the latter, as no external shell is present (except in rare and more modern groups), and the internal skeleton is not necessarily calcareous. Nevertheless,we have an ample record of the Cuttle-fishes of the Jurassic period, in the shape of the fossilised jaws or beak, the ink-bag, and, most commonly of all, the horny or calcareous structure which is embedded in the soft tissues, and is variously known as the "pen" or "bone." The beaks of Cuttle-fishes, though not abundant, are sufficiently plentiful to have earned for themselves the general title of "Rhyncholites;" and in their form and function they resemble the horny, parrot-like beak of the existing Cephalopods. The ink-bag or leathery sac in which the Cuttle-fishes store up the black pigment with which they obscure the water when attacked, owes its preservation to the fact that the colouring-matter which it contains is finely-divided carbon, and therefore nearly indestructible except by heat. Many of these ink-bags have been found in the Lias; and the colouring-matter is sometimes so well preserved that it has been, as an experiment, employed in painting as a fossil "sepia." The "pens" of the Cuttle-fishes are not commonly preserved, owing to their horny consistence, but they are not unknown. The form here figured (Beloteuthis subcostata, fig. 172) belonged to an old type essentially similar to our modern Calamaries, the skeleton of which consists of a horny shaft and two lateral wings, somewhat like a feather in general shape. When, on the other