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 Nerinoea, 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 Faxoee 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 (Cyproea), the Mitre-shells (Mitra), the Wing — shells (Strombus), the Scorpion-shells (Pteroceras), &c.

[Illustration:  Fig. 199.—­Hippurites Toucasiana.  A large individual, with two smaller ones attached to it.  Upper Cretaceous, South of Europe.]

[Illustration:  Fig. 200.—­Voluta elongata.  White Chalk.]

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 body-chamber, its simple septa between the air-chambers, and its nearly or quite central siphuncle.  The majority of the chambered Cephalopods of the Cretaceous belong, however, to the complex and beautiful family of the Ammonitidoe, with their elaborately folded and lobed septa and dorsally-placed siphuncle.  This family disappears wholly at the close of the Cretaceous period; but its approaching extinction, so far from being signalised by any slow decrease and diminution in the number of specific or generic types, seems to have been attended by the development of whole series of new forms.  The genus Ammonites itself, dating from the Carboniferous, has certainly passed its prime, but it is still represented by many species, and some of these attained enormous dimensions (two or three feet in diameter).  The genus Ancyloceras (fig. 202), though likewise of more ancient origin (Jurassic), is nevertheless very characteristic of the Cretaceous.  In this genus the first portion of the shell is in the form of a flat spiral, the coils of which are not in contact; and its last portion is produced at a tangent, becoming ultimately bent back in the form of a crosier.  Besides these pre-existent types, the Cretaceous rocks have yielded a great number of entirely new forms of the Ammonitidoe, which are not known in any deposits of earlier or later date.  Amongst the more important of these may be mentioned Crioceras, Turrilites, Scaphites,