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 (_Patellidoe_), the Nerites (_Nerita_), the _Turritelloe, Chemnitzioe_, &c., still hold a predominant place. The two most noticeable genera of this group are _Cerithium_ and _Nerinoea_--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 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 _Nerinoea_ 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_. [Illustration: Fig. 170.--_Ammonites Humphresianus_. Inferior Oolite.] [Illustration: Fig. 171.--_Ammonites bifrons_. Lias.]