206 CONIFBEALES [CH. 



features are similar to those associated with Cupressinoxylon, but 

 the xylem-parenchyma may not be always present and the medul- 

 lary-ray cells have one or two large simple pits, or two or more 

 smaller bordered pits, in the field, the pore being rather vertical 

 than horizontal. Mesembrioxylon undoubtedly includes species 

 which if additional data were available would be assigned to 

 distinct genera. Apart from the probability that anatomical 

 characters were even less restricted in their range through difierent 

 types in former periods than they are in existing genera, the 

 impossibility of discriminating between certain closely allied 

 recent Conifers points to the advisability of employing designations 

 for fossil woods in a wide sense and thus avoiding the danger of 

 misleading students in search of material on which to base con- 

 clusions with regard to the relative antiquity of existing genera. 



Mesembrioxylon sp. (= ? Podocarpoxylon sp. Gothan). 



This wood from Bathonian rocks in Russian Poland^ affords 

 an example of the difficulty of distinguishing clearly between 

 Podocarpoxylon and Glyptostroboxylon : the bordered pits on the 

 radial walls of the tracheids are separate or contiguous and 

 slightly flattened ; xylem-parenchyma occasionally occurs ; there 

 are usually two fairly large circular simple pits in the field (fig. 722, 

 C). The systematic position of this wood cannot be regarded as 

 well established. 



Miss Holden^ has recently described two specimens from the 

 Jurassic beds on the Yorkshire coast as Podocarpoxylon sp. but 

 the evidence in support of afiinity to the Podocarpineae is not by 

 any means conclusive. In one specimen there is no xylem- 

 parenchyma and in the other parenchyma occurs at the end of the 

 year's growth. The pits on the medullary rays are described as 

 piciform, 1 — 2 in the field. The anatomical features described 

 hardly afford adequate reasons for assigning the wood to Podo- 

 carpoxylon rather than to Cupressinoxylon. 



? Mesembrioxylon sp. (Thomas). 



A specimen of imperfectly preserved wood from Jurassic rocks 

 in the Izium district, in South Russia, described by Thomas^ as 



1 Gothan (00=) p. 456, fig. 5. ^ Holden, R. (13=) p. 542, PI. XL. figs. 31, 32. 



=> Thomas, H. H. (11) p. 80, PI. v. figs. 5—7. 



