The Nautiloid shell-fishes burst suddenly upon us in the beginning of the Siluro-Cambrian, or Lower Silurian, Barrande’s second fauna; and this applies to all the countries where they have been studied. In this formation alone about 450 species are known, and in the Silurian these increase to 1,200; and here the group culminates. It returns in the Devonian to about the same number with the Lower Silurian, diminishes in the Carboniferous to 350, and in the Mesozoic, where the Nautiloid forms are replaced by others of the type of the Ammonites, becomes largely reduced. In the Tertiary there are but nineteen species, and, as already stated, in the modern world _three_. These statements do not, however, represent the whole truth. In the Palæozoic, in addition to the genus _Nautilus_, we have a great number of other genera, some with perfectly straight shells, like _Orthoceras_ (Fig. 66), others bent (_Cyrtoceras_), others differing in the style of siphuncle, or aperture, or chambers (_Endoceras_, _Gomphoceras_, _Lituites_, Figs. 67 to 69), or inflated into sac-like forms (_Ascoceras_). There is, besides, the family of the _Goniatidæ_ (Fig. 70), with the chambers thrown into angular folds and the siphuncle at the back. Further, some of the early forms, as the Orthoceratidæ, attain to gigantic dimensions, being six feet or more in length, and nearly a foot in diameter. Thus the idea that we should naturally form from the study of the Nautilus, that it represents a type suited for much more varied and important adaptations than those that we now see, is more than realised in those Palæozoic ages when these animals seem to have been the lords of the seas. [Illustration: FIG. 66.—_Orthoceras_. Siluro-Cambrian. The dotted line shows the position of the siphuncle.] [Illustration: FIG. 67.—_Gomphoceras._] [Illustration: FIG. 68.—_Lituites._] [Illustration: FIG. 69.—_Nautilus Avonensis_ (Dn.). Carboniferous. _a_, Shell, reduced. _b_, Section, showing siphuncle.] [Illustration: FIG. 70.—_Goniatites crenistria_ (Philips). Carboniferous.] [Illustration: FIG. 71.—_Ceratites nodosus_ (Schloth). Triassic.]