16 



THE PSILOPHYTON FLORA 



[CH. 



Distribution. Devonian and perhaps Silurian. In Scotland 

 Psilophyton apparently occurs only in the Lower and Middle 

 Old Red. It however occurs in Upper Devonian rocks in the 

 United States and possibly in Belgium and Germany. It is thus 

 clear that the genus is distributed throughout Devonian times. 

 According to Dawson 1 it also occurs in Silurian rocks of Canada. 



Fig. 2. Psilophyton. Fertile fragments restored 

 by Reid and Macnair in 1896. a, b, fertile axis: 

 c, circulate vernation ; d, rhizomata. Considerably 

 reduced. After Reid and Macnair (1896). 



Several species of Psilophyton variously regarded as quite, or 

 as probably, distinct, occurring in Scotland and Canada, were 

 first discriminated by Dawson in 1859 or in 1871. Unfortunately 

 on the subject of species of this genus there is at present con- 

 siderable confusion and a critical revision of Psilophyton is now, 

 in the light of Rhynia, more than ever needed. The difficulty 

 here lies in the fact that it can only be attempted by some 



1 Dawson (1871). 



