2O8 HISTORICAL PALEONTOLOGY. 



last need special notice. The Cycads (fig. 140) are true exo- 

 genous plants, which in general form and habit of growth pre- 



Fig. 140. Zamia spirals, a living Cycad. Australia. 



sent considerable resemblance to young Palms, but which in 

 reality are most nearly related to the Pines and Firs (Coniftrai). 

 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 Ask, 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 Pterophyllum 

 (fig. 141, $), Zamites, and Podozamites (fig. 141, c\ 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, a\ which possesses some peculiar 

 characters, but would appear to be most nearly related to the 

 recent Cypresses. 



As regards the Invertebrate animals of the Trias, our know- 

 ledge is still principally derived from the calcareous beds 

 which constitute the centre of the system (the Muschelkalk) 



