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 a Nerinees.”  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.]

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 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, 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.