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INTRODUCTION 



IT is now clear that in Devonian times, two terrestrial floras, 

 quite distinct as regards affinity, existed, one in the earlier part, 

 and one in the later portion of the Devonian period. The former 

 will here be termed the Psilophyton flora ; it consisted largely, 

 as we hope to show, of Thallophyta belonging, for the most 

 part, to a group now quite extinct which we propose to term 

 the Procormophyta or Propteridophyta. The later flora con- 

 sisted chiefly, but not entirely, of plants which were obviously 

 Pteridophyta. This assemblage we propose to term the Archaeo- 

 pteris flora. 



Our knowledge of both these floras, though still far from 

 complete, has been entirely revolutionised during the last few 

 years by the publication, both at home and abroad, of a series 

 of memoirs, to which we shall presently refer more in detail. 

 These invaluable contributions have necessitated a complete 

 revision of the whole subject, and since no general account of 

 these floras, including these recent advances, at present exists, 

 we propose to commence by a brief enumeration of the characters 

 of their more important genera. We have purposely omitted 

 from our review all the more doubtful types about which little 

 or nothing is known beyond the existence of very obscure or 

 fragmentary examples. We have further in Chapter III of this 

 book confined our attention to a critical summary of the essential 

 features of the morphology and anatomy of the genera belonging 

 to the Psilophyton flora, reserving for a separate chapter (p. 46) 

 the entire discussion of the question of their affinities and 

 systematic position. Finally in yet a further Chapter we discuss 

 the very important bearing of these new discoveries on the 

 phylogeny of Cormophyta and in particular of the various 

 Pteridophytic lines of descent. 



The researches to which we particularly refer are firstly 

 Kidston and Lang's 1 memoir on the Scottish plant Rhynia 



1 Kidston and Lang (1917). 



