and the Rhætic or Kössen beds were slowly accumulated in an open sea, of at least a moderate depth; and they have preserved for us a very considerable selection from the marine fauna of the Triassic period. [Illustration: Fig. 140.--_Zamia spiralis_, a living Cycad. Australia.] The _plants_ of the Trias are, on the whole, as distinctively Mesozoic in their aspect as those of the Permian are Palæozoic. In spite, therefore, of the great difficulty which is experienced in effecting a satisfactory stratigraphical separation between the Permian and the Trias, we have in this fact a proof that the two formations were divided by an interval of time sufficient to allow of enormous changes in the terrestrial vegetation of the world. The _Lepidodendroids, Asterophyllites_, and _Annularioe_, of the Coal and Permian formations, have now apparently wholly disappeared: and the Triassic flora consists mainly of Ferns, Cycads, and Conifers, of which only the two last need special notice. The _Cycads_ (fig. 140) are true exogenous plants, which in general form and habit of growth present considerable resemblance to young Palms, but which in reality are most nearly related to the Pines and Firs (_Coniferoe_). 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 remains of Cycads belonging to such genera as _Pterophyllum_ (fig. 141, b), _Zamites_, and _Podozamites_ (fig. 141, c), are sufficiently 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 _Veltzia_ (fig. 141, a), which possesses some peculiar characters, but would appear to be most nearly related to the recent Cypresses. [Illustration: Fig. 141.--Triassic Conifers and Cycads. a, _Voltzia_ (_Schizoneura_) _heterophylla_, portion of a branch, Europe and America; b, Part of the frond of _Pterophyllum Joegeri_, Europe; c, Part of the frond of _Podozamites lanceolatus_, America.]