596 



GEOLOGY. 



The frond-genera belong in large part to the stem-genera, the fronds 

 of the I^yginodendron being sphenopterids, those of Medullosa, neurop- 

 terids and alethopterids, and so forth; but the full association of fronds 

 and stems has not yet been determined. How nearly the group grades 

 into the true ferns is not known, but in a general way it constitutes a 

 transition series between the ferns and cycads, and is apparently an 

 illustration of a radical evolution in progress. Initial forms have 

 been identified from the Devonian and Subcarboniferous, and a few 



. 



Fig. 278. — A typical Cycadofilices, Lyginodendron oldhamia. 



Scott and J. Allen.) 



(Restoration by D. H. 



are found in the Permian; but the group is essentially Carboniferous, 

 which suggests that probably the evolution was mainly accomplished 

 between the Devonian and the close of the Carboniferous. 



The Equisetales (calamites, horsetails). — What is now the lowly 

 tribe of horsetails with a single genus, Equisetum, constituted a marked 

 feature in the Carboniferous flora, in the form of Calamites. These 

 calamarians not only attained much greater sizes, but much higher 

 and more varied organizations than the present equiseta. Even the 

 largest tropical members of to-day, on the eastern slope of the 



