Fig. 69.—Nautilus Avonensis (Dn.). Carboniferous.
a, Shell, reduced. b, Section, showing siphuncle.
Fig. 70.—Goniatites crenistria (Philips). Carboniferous.
Fig. 71.—Ceratites nodosus (Schloth). Triassic.
When we leave the Palæozoic and enter the Mesozoic, though the Nautiloid shells still abound, we find them superseded, in great part, by a nobler form, that of the Ammonitidæ ([Figs. 71, 72]). These are remarkable for the ornate markings on the surfaces of their shells, and for the beautifully waved edges of the partitions ([Fig. 72]a), which, by giving a much more complete support to the sides of the shell, must have contributed greatly to the union of lightness and strength so important to the utility of the shell as a float. This type admits of all the same variety of straight, bent, and curled forms with the simpler Nautiloid type, and some of the species are of great size, Ammonites being known three feet or more in diameter. These animals, unknown in the Palæozoic, appear in numerous species in the Early Mesozoic, culminate in hundreds of beautiful species in the middle of that era, and disappear for ever at its close, leaving no modern successors. Many and beautiful species of Ammonites and their allies have been obtained from the Mesozoic rocks of British Columbia and other parts of the west coast of North America, perfectly representing this group as it occurs at the same period in Europe, and closely resembling the Mesozoic Ammonites of India. These animals have all perished, yet the Atlantic and the Pacific roll between, apparently with conditions as favourable for their comfortable existence as those of any previous time. They perished long ago, at the dawn of the Tertiary; yet the genus Nautilus, one of the oldest and least improved of the whole, survived, and still testifies to the wonderful contrivance embodied in these animals.