40 



THE PSILOPHYTON FLORA 



[CH. 



possibly no longer exists, in fact as a "Thallophyte with Algal 

 affinities 1 ." 



As to whether Parka, as here described, represents the whole 

 plant, there must always remain a slight doubt, until petrified 

 specimens are known. This uncertainty however appears to us 

 to be very slight, and Don and Hickling find no evidence that 

 Parka represented "aquatic plants with creeping stems, linear 

 leaves" as had been asserted by Daw r son and Penhallow 2 as late 

 as 1891. They have also failed to discover any evidence of the 

 prothalli and heterospores which the earlier workers believed 

 they could recognise. 



-<3 ! 1 3 3 ;\C3 "3 g* n i j >*T fSs^ 



; 



Fig. 21. Living Coralline Algae for comparison with Parka. (1) L?7/o- 

 phyllum lichenoides, (E. & S.), external morphology (nat. size). (2) Melobesia 

 membranacea (Esper) Lamour, vertical section through a conceptacle contain- 

 ing tetraspores ( x 350). Both after Rabenhorst's Kryptogamen- Flora (1885). 



There can be no doubt that Don and Hickling's work on 

 Parka -has removed a cloud of doubt and suspicion in regard 

 to this fossil, much of which appears to have been ill-founded. 

 Several points will remain uncertain until petrified material is 

 discovered. All that has so far been made out of the structure 

 of the thallus has been accomplished by means of macerating 

 carbonised and cuticularised impressions. At the same time the 

 important conclusion that we are dealing here with a lowly 

 Thallophyte, not far removed from the Algae, and even com- 

 parable in habit to certain living Coralline Algae, is one which 

 is not likely to be displaced, but rather to be confirmed, by a 

 knowledge of structure material. 



1 Don and Hickling (1917), p. 661. 



2 Dawson and Penhallow (1891), p. 16. 



