MANUAL OF NATURAL HISTORY. 523 



especially in the carboniferous series. Those most 

 abundantly met with are Filices, JEquisetacece, and 

 Lycopodiacece, the latter of gigantic dimensions ; 

 also a few mosses and sea-weeds. In more recent 

 formations, the proportional numbers of Cryptoga- 

 mia are much lessened ; and in the later deposits, 

 the various classes of fossil vegetables bear nearly 

 the same ratio to each other as in the existing 

 Flora, 



II— Sub-Kingdom. Flowerless-Plants 

 (Cryptogamia). 



Flowers wanting ; fructification by means of 

 spores ; sexes wanting or indistinct ; germination 

 heterorhizal. 



ACROGENS. 



The more advanced cryptogamic plants, into the 

 composition of which vascular tissue enters, and 

 which, in many respects, approach some Phanero- 

 gamia, have been separated, as a distinct class, 

 under the name of Acrogens, or Cormogens. In 

 them a stem and leaves can be distinguished ; flowers 

 are absent, but they seem to be represented in 

 some instances, by the mode in which the leaves are 

 arranged round the spore-cases of Urn-mosses, or by 

 the involucrate membrane surrounding the thecce of 

 some Liver-worts, Among their reproductive organs 

 two forms are generally present, which many writers 

 distinguish as male and female, under the names of 



