XLIV] MESEMBRIOXYLON 205 



separate and are smaller than in Xenoxylon. The pits on the 

 medullary-ray cells are also smaller, though it is doubtful if this 

 is a constant character. Miss Holden^ has instituted a new generic 

 name, Paraphyllocladoxylon, for two specimens of wood from 

 Jurassic rocks on the Yorkshire coast which do not appear to 

 differ from Mesembrioxylon in any respect calling for generic 

 recognition. In Paraphyllocladoxylon eboracense, from the Oolite 

 of Scarborough, the tracheids have usually scattered and circular 

 pits on the radial walls and pits are also abundant on the tangential 

 walls: in Paraphyllocladoxylon araucarioides the pits on the radial 

 walls are always closely compressed and flattened. There is no 

 Abietineous pitting and there may be one or occasionally two 

 large simple pores in the field like those in Xenoxylon and Meserti- 

 brioxylon (= Phyllocladoxylon of Gothan) but smaller than those 

 of Xenoxylon. Xylem-parenchyma is absent, but some tracheids 

 have apparent cross-walls that are believed to be resin-plates. 

 Miss Holden recognises the close resemblance of her species to 

 Gothan's Phyllocladoxylon, but a new name is employed on the 

 ground that the absence of Sanio's rims shows that the wood of 

 the Yorkshire plants is Araucarian. The absence of Sanio's rims 

 cannot be confidently regarded as an original feature and, assuming 

 .this negative character to be a real one, it does not differentiate 

 the specimens from those described by Gothan; Gothan's figures 

 afford no evidence of the presence of Sanio's rims in his species of 

 Phyllocladoxylon. If the Yorkshire stems are Araucarian so too are 

 those from King Charles Land and Seymour Island^. In one of 

 Miss Holden's species the tracheal pitting is not of the Araucarian 

 type, while in the other it is Araucarian ; the pitting of the medullary 

 rays is opposed to an affinity to any recent Araucarian Conifer. 

 Both of the Yorkshire species are therefore transferred to Mesem- 

 brioxylon: their anatomical characters indicate that they are 

 generalised types which cannot legitimately be included in any 

 family based solely on existing Conifers. While recognising that it 

 is not always easy to draw a definite distinction between Xenoxylon 

 and Mesembrioxylon the two names may be conveniently retained, 

 the former being used in a much more restricted sense than the 

 latter. Mesembrioxylon is applied to woods in which the general 



1 Holden, R. (13=) p. 536, PI. xxxix. figs. 7—10. " Gothan (07^); (08). 



