CTCADACE2E. .3 



4 



tation of former periods. The recent cycads are usually divided 

 into nine genera and two families : the Cycadece, including one 

 genus, Cycas ; and the Zamiece, with the genera Zamia, Cerato- 

 zamia, Macrozamia, Dioon, Encephalartos, Stangeria, Bowenia, and 

 Microcycas. 1 



None of the living cycads occur outside tropical or subtropical 

 regions. In Tertiary times the family does not appear to have 

 had a wide distribution, nor to have been represented by many 

 genera ; possibly, however, a closer acquaintance with extra- 

 European Tertiary strata may bring to light a greater number 

 of cycads from these beds than are at present known. In the 

 Mesozoic period cycads occupied a prominent position, and had 

 an extended geographical range. The Jurassic strata afford 

 abundant evidence that cycadean plants reached their maximum 

 development in that era ; less numerous in the Triassic vegetation, 

 the Cycadacecs dwindle down to a few representatives in the 

 Permian and Carboniferous floras. 



Before giving a summary of the earlier geological history of this 

 exceedingly interesting section of the Gymnospermce, we may take 

 note of some of the difficulties which beset any attempt to trace the 

 geological history of cycadean plants. As in the case of ferns, and 

 indeed of all fossil plants, so here again we have to deal in nearly 

 every instance with detached and isolated specimens of stems, 

 fronds, flowers, and seeds. The fronds are often abundant enough, 

 and their preservation frequently good ; but the characters which 

 are made use of in generic and specific determinations are such as 

 preclude any certain conclusions as to precise botanical affinity. 

 The nature of cycadean flowers, and their manner of occurrence 

 on the plant, separated as they are from the sterile fronds, present 

 an obstacle to exact determination. On the other hand, the fronds 

 alone afford, in many instances, convenient data on which to found 

 a provisional classification ; their form and general habit of growth 

 are fairly uniform, and they do not present the same striking 

 variation in leaf form which constitutes one of the many difficulties 

 associated with the fronds of fossil ferns. Among recent cycads 

 we have a phyllopodium exhibiting, in the majority of species, 

 certain distinct and easily recognized characters ; usually a pinnate 

 structure, with stout and more or less closely set segments 



1 Engler and Prantl, Teil ii. p. 6. See also De Candolle. 



