208 HISTORICAL PALHZONTOLOGY. 
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 spiralis, a living Cycad. Australia. 
sent considerable resemblance to young Palms, but which in 
reality are most nearly related to the Pines and Firs (Conzfere). 
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 acone. 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 Pterophyllum 
(fig. 141, 0), 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 Voltsia (fig. 141, a), which possesses some peculiar 
characters, but would appear to be most nearly related to the 
recent Cypresses. 
As regards the /uzvertebrate animals of the Trias, our know- 
ledge is still principally derived from the calcareous beds 
which constitute the centre of the system (the Muschelkalk) 
