506 Notices of Memoirs — 



whicli reached a diameter of 2 or 3 feet — exhibit a regular variation 

 in width, giving the appearance of concentric rings of growth, as in 

 the stems of the tree-like Lessonia, an existing genus of Antarctic 

 seaweeds. This structural feature presents an impressive image 

 in stone of a plant's rhythmical response to some periodically 

 recurring conditions of growth in the waters of Palgeozoic seas. 



Devonian and Lowek Carboniferous Floras. — The earliest 

 plants that have been found in sufficient number, and in a state 

 of preservation which renders their identification possible, are those 

 from Devonian rocks. 



AVhat do we know as to the composition of the floras that 

 flourished in the later stages of the Devonian and in the lower part 

 of the Carboniferous era? The following list, which is by no 

 means exhaustive, represents some of the more important generic 

 types which may be very briefly described : — 



1. EaUISETALES. 



ulrcJiceocalamites. 



2. Sphenophyllales. 



Sphenophyllmn. 

 Cheirostrohus. 

 \_PseHcloboronia ?] 



3. Lycopodiales. 



Lepidodenclron. 

 Bothrodenclron. 



4. FiLICALES. 



Arch(eopter\s. 

 Adiantites. 



Rhodea. 



Cardiopteris. 

 Todeopsis. 

 Cephalotheca. 

 Rhacopteris. 



5. Cycadofilices. 



Calamopitys. 



Heterangimn. 



Lyginodendron. 



6. Gymxospermje. 



(cosdaitales.) 



Cordaites. 



Fitys. 



In ArcJitsocalamites we have the oldest example of an undoubted 

 Equisetaceous genus. The structure of its comparatively thick and 

 woody stem is practically identical with that of our common British 

 type of Calamites, one of the most abundant of the Coal-period 

 genera, while the strobilus difi'ered in no essential feature from that 

 of a modern Horsetail. The genus Cheirostrohus, founded in 1897 

 by Dr. D. H. Scott on a single specimen of a petrified cone, affords 

 a striking illustration of a Palgeozoic plant exhibiting a structure far 

 more complex than that of any known type among existing Vascular 

 Cryptogams. In this Scotch cone, about 3-0 cm. in diameter, we 

 recognise Equisetaceous and Lycopodinous characters combined with 

 morphological features typical of the extinct genus Sphenophyllum. 

 Both Devonian and Culm rocks have furnished many examples of 

 Lycopodinous plants. The genus Botlirodendron, closely allied in 

 habit to Lepidodendron, has been recorded from Bear Island, Ireland, 

 and Australia, and the cuticles of a Lower Carboniferous species 

 form the greater portion of the so-called paper-coal of Tula in 

 Eussia. Lepidodendron itself had already attained to the size of 

 a forest tree, with anatomical features precisely similar to those 

 of the succeeding Coal-period species. 



Our knowledge of the ferns is not very extensive. The genus 

 Archceopteris from Ireland, Belgium, Bear Island, and North America 



