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. Within these limits, each formation is characterised by particular species, the number of individuals being often very great, and the size which is sometimes attained being nothing short of gigantic. In the Lias, particular species of _Ammonites_ may succeed one another regularly, each having a more or less definite horizon, which it does not transgress. It is thus possible to distinguish a certain number of zones, each characterised by a particular Ammonite, together with other associated fossils. Some of these zones are very persistent and extend over very wide areas, thus affording valuable aid to the geologist in his determination of rocks. It is to be remembered, however, that there are other species which are not thus restricted in their vertical range, even in the same formations in which definite zones occur. [Illustartion: Fig. 172.--_Beloteuthis subcostata_ Jurassic (Lias).]