. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. 208 HISTORICAL PALEONTOLOGY. last need special notice. The Cycads (fig. 140) are true exo- genous plants, which in general form and habit of growth pre-. mn' Fig. 140.—Zainia spiralis, ^ilyc'SiA. Australia. sent considerable resemblance to young Palms, but which in reality are most nearly related to the Pines and Firs {Coniferce). The trunk is unbranched, often much shortened, and bears a crown of feathery pinnate fronds. The leaves are usually &qu
. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. 208 HISTORICAL PALEONTOLOGY. last need special notice. The Cycads (fig. 140) are true exo- genous plants, which in general form and habit of growth pre-. mn' Fig. 140.—Zainia spiralis, ^ilyc'SiA. Australia. sent considerable resemblance to young Palms, but which in reality are most nearly related to the Pines and Firs {Coniferce). The trunk is unbranched, often much shortened, and bears a crown of feathery pinnate fronds. The leaves are usually " circinate ^' — they unroll in expanding, like the fronds of ferns. The seeds are not protected by a seed-vessel, but are borne upon the edge of altered leaves, or are carried on the scales of a cone. All the living species of Cycads are natives of warm countries, such as South America, the West Indies, Japan, Australia, Southern Asia, and South Africa. The remains of Cycads, as we have seen, are not known to occur in the Coal formation, or only to a very limited extent towards its close; nor are they known with certainty as occurring in Permian deposits. In the Triassic period, however, the re- mains of Cycads belonging to such genera as Fterophyllum (fig. 141, b), Zainites^ and Podozaviites (fig. 141, ^), are suffi- ciently abundant to constitute quite a marked feature in the vegetation; and they continue to be abundantly represented throughout the whole Mesozoic series. The name " Age of Cycads,'' as applied to the Secondary epoch, is therefore, from a botanical point of view, an extremely appropriate one. The Conifers of the Trias are not uncommon, the principal form being Voltzia (fig. 141, ^), which possesses some peculiar characters, but would appear to be most nearly related to the recent Cypresses. As regards the Invertebrate afiimals of the Trias, our know- ledge is still principally derived from the calcareous beds which constitute the centre of the syst
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