The Bivalves, which had already become such important elements of the Triassic fauna, greatly increase in the Jura, their shells forming great banks and strata. Many of the genera are still living, and only a few of the more abundant ones can be mentioned here. Oysters like Gryphcea (XIV, 13), Exogyra, and Ostrea itself (XIV, 14) are common, and the Scallop shell, Camp-tonectes (XIV, 10), is important. Trigonia (Fig. 288 and PL XIV, Fig. 8) is especially characteristic of the Jura, but a few representatives of that genus have persisted to the present time and are found in the Australian seas. Diceras and Pholadomya (XIV, 9) are likewise common genera, and there are very many others. Among the Gastropoda the most significant change lies in the importance which the siphon-mouthed shells now for the first time assume; examples of this group are Nerinea (XIV, 17), Alaria, Purpurina, etc. Of the shells with entire mouths the ancient Palaeozoic genus Pleurotomaria (XIV, 16) is as abundant as ever, not beginning to decline until the Cretaceous period.
Fig. 288. - Slab of Trigonia clavellata, from the English Oxfordian.
The Cephalopods are at the very height of their culmination, and are present in an astonishing profusion and diversity, filling whole strata with their heaped-up shells. The Nautiloids differ from those of the Trias in their smoother and more involute shells. The Ammonoids do not display so many types of shell structure as we have found in the Trias, and the genera are mostly different from those of the latter period; but in number of distinct species the Jura much surpasses the other Mesozoic periods. Phylloceras and Lytoceras (XIV, 19) continue on from the Trias, but the most abundant, characteristic, and widely spread genera are new. Of these may be mentioned: Peltoceras (XIV, 18) Arietites, Aegoceras, Harpoceras, Stephanoceras, Perisphinctites, and many others, each with large numbers of species. Crioceras (XIV, 20) is an uncoiled ammonoid. The Belemnites (Fig. 289), which were introduced in a small way in the Trias, in the Jurassic blossom out into an incredible number of forms, exceeding even the Ammonites in abundance of individuals, if not of species.
These extinct Ce-phalopods belonged to the Dibranchiata, as do all the living forms except the Pearly Nautilus; they in some measure serve to connect the extinct genera having external shells with the existing naked squids and cuttle-fishes, which have only rudimentary internal shells, the pen or cuttle-bone. The Belemnites have a straight, conical, chambered shell, called the phragmocone, which ends above in a broad, thin plate. The phragmocone was partly external to the animal, and its lower, pointed end was inserted into a dart-or club-shaped body called the guard or rostrum which is composed of dense, fibrous, crystalline calcite. Usually only the guard is preserved in the fossil state, and specimens are so common that they have attracted popular interest and bear the folk-name of "thunderbolts." In a few instances the animal has been preserved almost entire, so that the structure is well understood.
Fig. 289. - Slab of Belemnites compressus Blainv, from the Lias of England.