FOSSIL FORMS 



275 



organ. In the lowest strata the plants are to be found associated 

 with remains of marine animals, thus suggesting their power to 

 grow under marine or brackish conditions, whilst in the higher 

 strata they occur in beds, which are regarded as fresh- water or 

 continental, where they are associated with plants that were un- 

 doubtedly terrestrial. The presence of spores in Nematothallus is 

 regarded as rendering it unlikely that they were algal in nature, 

 but the spores may be comparable to the hard-walled cysts such as 

 are to be found in Acetabularia. 



Fig. 178. Nematophytales. Nematophyton, A, longitudinal section (X120). 

 B, transverse section ( x 120). (After Seward.) 



The genus Nematophyton is found in the Silurian and Devonian 

 rocks where it was first described under the name of Prototaxites 

 and referred to the Taxaceae, but subsequently it was accepted as 

 an alga and renamed Nematophyton or Nematophycus. Later the 

 name Prototaxites was revived and it was placed in the Phaeo- 

 phyceae, whilst Krausel (1936) recently stated that it must have 

 had the appearance of a Lessonia (cf. p. 180) and also that it existed 

 in aquatic habitats which may have been marine, brackish or fresh. 

 The vahd name is therefore Prototaxites, but as this tends to convey 

 a false impression of the plant's affinities it would seem more 



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