20 THE PSILOPHYTON FLORA [ch. 



and a smooth type with characteristic costation and no evi- 

 dence of spines or scales. The latter is a more lax type, and freely 

 branched. 



The most recent work bearing on this genus is that of Halle 

 on Lower Devonian specimens from Norway, and of Kidston 

 and Lang on the Scottish Devonian Rhynia. We will now con- 

 sider these very important contributions in some detail beginning 

 with Halle's conclusions. 



Halle^, in an elaborate attempt to apply some definite 

 meaning to the term Psilophyton — an attempt with which at 

 the time it was written we were much in sympathy — would wish 

 to confine this term to those stems alone which, in whole or 

 part, bear spine or leaf -like organs. He says " in order to establish 

 an acceptable definition of the genus Psilophyton, it is necessary 

 to confine its use to stem-like structures bearing spines or small 

 leaves. Isolated branch-systems without spines... cannot be 

 regarded as belonging to Psilophyton unless they are found in 

 actual connection with spine-bearing Psilophyton-stems^." The 

 non-spinous stems bearing fructifications (Fig. 6), which Dawson 

 referred to P. princeps, are removed by Halle to a distinct genus 

 Dawsonites, as D. arcuatus n. spec. ; according to HaUe the term 

 P. princeps should be used only for the spiny type of stem (Fig. 

 3), the fructification of which he asserts is not as yet known. 



Throughout Halle's criticisms it is clear that he shared in no 

 small degree the doubt which others had long cast on the 

 correctness of Dawson's morphological and taxonomic con- 

 clusions. It is perfectly true of course that Dawson did not 

 prove, by means of incontestable figures, many of his state- 

 ments, in the manner which we have learnt to expect in modern 

 research. It has also to be borne in mind that when Halle wrote 

 his memoir, he did not know of the entirely new light which 

 Rhynia has since shed on these questions. 



It must however be confessed that even at that time there 

 were no just grounds for discriminating species merely on the 

 presence or absence of macroscopic scale or spine-like emergences. 

 Several species ot Psilophyton {P. rohustius, P. elegans, etc.) were 

 already known in which no such emergences are found, and thus 

 1 Halle (1916). ^ Ibid. p. 22. 



