CHAPTER II 



THE TWO DEVONIAN FLORAS 



As we have already stated, it is clear that two distinct floras 

 existed during Devonian times, not side by side, but successively. 

 This is clear from a comparison of the dominant genera of the 

 three horizons in that series. Among the earlier types, such 

 genera as Psilophyton, Arthrostigma and Hostimella^ are pro- 

 minent and many of the plants of the Upper Devonian period 

 are entirely wanting. This we propose to term the Psilophyton 

 flora. It was a flora not by any means sharply marked off from 

 that which preceded, or that which succeeded it. In the earlier 

 Devonian rocks, we find not only the dominant members of 

 this type of flora, but also survivals of a still earlier flora of 

 which Cryptoxylon, Nematophycus and Pachytheca are examples. 

 We do not say that these types are ever found in the same beds 

 as Psilophyton — all we remark is, that they existed at the same 

 period. 



The land flora of Silurian times is at present almost unknown, 

 but we are acquainted, in Parka, with at least one British genus 

 of the Psilophyton flora, Avhich goes back as far as the Silurian. 

 It has been also stated by Dawson that Psilophyton itself occurs 

 in the Upper Silurian of Canada. It would thus seem that the 

 flora of the close of Silurian times, whether marine or terrestrial, 

 had much in common with that of the earliest stage in Devonian 

 history. We know of at least four genera common to these two 

 formations. 



As we pass upwards from the lowest sediments of Devonian 

 age, we find that the members of the Psilophyton flora begin to 

 die out and that their place is taken by new arrivals, which, as 



^ [The spelling HostimeUa and not IlostineUa is used here, since Jahn 

 (1903), p. 74, shows that the former is correct. See also Potonie, H. and 

 Bernard, C. (1904), p. 11. A. A.] 



