18 



THE PSILOPHYTON FLORA 



[CH. 



The difficulty in regard to Dawson's species Avill, 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 jyrinceps 

 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 structiu'cs 

 which Dawson himself lu'ged. 



He states 1 explicitly that 

 most observers would separate 

 specifically the two types P. 

 princeps and P. ornatum, but he 

 believes they pass into one 

 another and cannot be clearly 

 separated on these grounds. 

 We may therefore enumerate 

 the species of impressions of 

 Psilophyton occurring in Scot- 

 land as follows: 



(1) P. princeps, Daws.^ (in- 

 cluding the variety P. ornatuin, 

 Erect shoots, slender or of medium thick- 

 or laterally branched, forks wide, bark 

 macroscopically smooth or covered with scattered, or numerous 



Fig. 4. Psilophyton princeps, Daw- 

 son, from the Lower Devonian of 

 Roragen, Norway. 1. Attributed 

 by Halle to the genus Arthro- 

 stigina, but described as a "narrow, 

 PsilophytonAike stem." 2. Part of 

 "1" twice enlarged to show nerves 

 of emergences. 3. Attributed by 

 Halle to Psilophyton princeps, or 

 possibly to Arthrosiigma. After 

 Halle (1916). 



Daws.3) (Figs. 3-5). 

 ness, dichotomously 



1 Dawson (1871), p. 39. '' 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. 



