Fig. 197.—Spondylus spinosus. White Chalk. (fig. 199) were attached to foreign objects, and lived associated in beds, like Oysters. The two valves of the shell are always
Fig. 198.—Inoceramus sulcatus. Gault. altogether unlike in sculpturing, appearance, shape, and size; and the cast of the interior of the shell is often extremely unlike the form of the outer surface. The type-genus of the family is Hippurites itself (fig. 199), in which the shell is in the shape of a straight or slightly-twisted horn, sometimes a foot or more in length, constituted by the attached lower valve, and closed above by a small lid-like free upper valve. About a hundred species of the family of the Hippuritidœ are known, all of these being Cretaceous, and occurring in Britain (one species only), in Southern Europe, the West Indies, North America, Algeria, and Egypt. Species of this family occur in such numbers in certain compact marbles in the south of Europe, of the age of the Upper Cretaceous (Lower Chalk), as to have given origin to the name of "Hippurite Limestones," applied to these strata.
The Univalves (Gasteropods) of the Cretaceous period are not very numerous, nor particularly remarkable. Along with species of the persistent genus Pleurotomaria and the Mesozoic
Fig. 199.—Hippurites Toucasiana. A large individual, with two smaller ones attached to it. Upper Cretaceous, South of Europe.
Fig. 200.—Voluta elongata. White Chalk. Nerinœa, we meet with examples of such modern types as Turritella and Natica, the Staircase-shells (Solarium), the Wentle-traps (Scalaria), the Carrier-shells (Phorus), &c. Towards the close of the Cretaceous period, and especially in such transitional strata as the Maestricht beds, the Faxöe Limestone, and the Pisolitic Limestone of France, we meet with a number of carnivorous ("siphonostomatous") Univalves, in which the mouth of the shell is notched or produced into a canal. Amongst these it is interesting to recognise examples of such existing genera as the Volutes (Voluta, fig. 200), the Cowries (Cyprœa), the Mitre-shells (Mitra), the Wing - shells (Strombus), the Scorpion-shells (Pteroceras), &c.
Upon the whole, the most characteristic of all the Cretaceous Molluscs are the Cephalopods, represented by the remains of both Tetrabranchiate and Dibranchiate forms. Amongst the former, the long-lived genus Nautilus (fig. 201) again reappears, with its involute shell, its capacious