XLIV] DADOXYLOX 179 



Araucarias as he examined have no pits even on the radial walls 

 leads one to suspect the accuracy of some of the many recorded 

 instances of fossil Araucarian wood with medullary-ray pits ; but 

 this does not affect the value to be attached to the presence or 

 absence of well-defined pits on the vertical or horizontal walls of 

 medullary-ray cells. It is generally true that in the ray cells of 

 Abietineae these walls are pitted while in the Araucarineae they 

 are unpitted, and it is equally true that the predominance of 

 alternate and contiguous pits on the tracheids is evidence of Arau- 

 carian affinity. Though geneially absent from Araucarian wood 

 xylem-parenchyma occasionally occurs in stems otherwise iden- 

 tical with the usual Araucarian type. The genus Araucariopsis 1 

 was instituted by Caspary for specimens distinguished from most 

 examples of Dadoxylon by the presence of scattered xylem-paren- 

 chyma but, as Gothan 2 points out, a distinctive name is super- 

 fluous; the type-species of Araucariopsis, A. macractis, should be 

 transferred to Dadoxylon ; in the possession of xylem-parenchyma 

 it agrees with Dadoxylon septentrionak Goth, from Spitzbergen. 

 The importance attached by Jeffrey and other American authors 

 to the presence of Sanio's rims on the tracheal walls is, I venture 

 to think, greatly overestimated. The determination of fossil wood 

 is to a large extent a question of relative values. There is clear 

 evidence, and it would be surprising were it not so, that in several 

 extinct types there are combinations of character pointing to less 

 sharply defined boundaries than those which delimit existing 

 families and genera. It is in the conclusions drawn from general- 

 ised types that authors differ. An outstanding fact is the pre- 

 dominance in Palaeozoic stems of the Araucarian form of tracheal 

 pitting which is unquestionably a much older type than that 

 characteristic of the Abietineae. The following definition is based 

 on specimens agreeing in the sum of their characters with recent 

 Araucarineae, but there are various genera described by Jeffrey, 

 Miss Holden, and other authors and believed by them to be more 

 or less closely allied to the Araucarineae which are not provided 

 for in the definition. These genera are treated separately as 

 generalised types and the decision as to the nature of the evidence 



1 Caspary and Triebel (89) p. 81. Pis. xiv., xv. 



2 Gothan (10) p. 9. 



122 



