18 



THE PSILOPHYTON FLORA 



[CH. 



The difficulty in regard to Dawson's species will, we believe, 

 vanish, if it can be shown, as we shall attempt to demonstrate 



here, that the presence or ab- 

 sence of macroscopic emergences 

 or so called spines is a matter 

 of no systematic importance. 

 The species Psilophyton princeps 

 should include those erect shoots 

 with fairly stout spine-like emer- 

 gences (Fig. 3), those on which 

 the emergences are small delicate 

 structures (Figs. 4 and 5) and, 

 further, those stems which are 

 apparently smooth and without 

 macroscopic emergences of any 

 kind. This it may be remarked 

 is exactly the view of the im- 

 portance of these structures 

 which Dawson himself urged. 



He states 1 explicitly that 

 most observers would separate 

 specifically the two types P. 



princeps and P. ornatwn, but he 



Fig. 4. Psilophyton princeps, Daw- , v , -, . , 



son, from the Lower Devonian of belie ^es they pass into one 



Roragen, Norway, i. Attributed another and cannot be clearly 



by Halle to the genus Arthro- separated on these grounds. 



stigma but described as a "narrow, We therefore enumerate 



Psilophyton-liKe stem. 2. Part of . . 



" 1" twice enlarged to show nerves the species of impressions of 



of emergences. 3. Attributed by Psilophyton occurring in Scot- 



Halle to Psilophyton princeps, or j an( j as follows : 

 possibly to Arthroslisma. 

 Halle (1916). 



After 



/-, \ -r / 



W P ' Pnceps, Daws. 2 (in- 



cluding the variety P. ornatum, 

 Daws. 3 ) (Figs. 3-5). Erect shoots, slender or of medium thick- 

 ness, dichotomously or laterally branched, forks wide, bark 

 macroscopically smooth or covered with scattered, or numerous 



1 Dawson (1871), p. 39. 2 Dawson (1871), PI. IX, figs. 102-108. 



3 Dawson (1871), PI. IX, figs. 97-101, 104, 104 a, 109-110. PI. X, figs. 

 112-114, 118. 



