SHOWING STRUCTURE, FROM THE RHYNTE CHERT BED, ABERDEENSHIRE. 779 



appeared in text-fig. 2. The "woody cylinder" or "woody fibres" (b in text-figs. 

 1 and 2) would correspond to the phloem of Rhynia, while the " scalariform axis" 

 {a, text-figs. 1 and 2) is the equivalent of the central strand of xylem. To make this 

 interpretation clear, we have added letters below text-fig. 1 corresponding to the 

 lettering used throughout our plates. This text-figure may further be compared 

 with the longitudinal sections shown in figs. 26, 42, and 59 of this paper. 



If this interpretation be correct, there would be a substantial agreement in 

 structure between Psilophyton princeps arid Rhynia Gwynne-Vaughani. The two 

 plants further agree in bearing large oval sporangia on the ends of ultimate branches 

 of the stem. Psilophyton princeps differs from Rhynia, however, in the presence of 

 spines, in. the more profuse dichotomous branching, in the subordination of some of 

 the branches to a sympodial main axis, and in the absence, so far as we know, of 

 lateral adventitious branches. 



On these grounds we regard Psilophyton as exhibiting characters which ally it 

 with the more fully known Rhynia, and separate these two genera from all other 

 vascular plants. The differences b- 'tween Rhynia and Psilophyton, however, warrant 

 their being treated as distinct genera. 



It will be clear that Rhynia and Psilophyton belong to the Vascular Cryptogams 

 or Pteridophyta, but they ca not be placed in any of the Classes of this great group 

 as at present known and defined. These Classes are the Filicales, Equisetales, 

 S j>li enophyllales , Psilotales, and Lycopodiales. 



It is therefore necessary to recognise another group of Pteridophyta, of equivalent 

 value to those mentioned, to include Rhynia Givynne-Vaughani and certain of the 

 specimens described under the name of Psilophyton princeps. This Class is 

 characterised by the sporangia being borne at the ends of certain branches of the 

 stem without any relation to leaves or leaf like organs. For this Class we propose 

 the name Psilophy tales. This name is derived from that of the earlier described 

 though less perfectly known genus Psilophyton, and further suggests the resemblance 

 between the plants of this class and the existing Psilotales. Whether the other 

 characters in which Rhynia and Psilophyton seem to agree will prove to be common 

 characters of the whole group, must be left open for the present, but they are not 

 essential for its definition. 



