178 CONIFBEALES [CH. 



Dadoxylon australe Arberi based on Palaeozoic wood from 

 Australia : the substitution of Dadoxylon for Araucarioxylon, the 

 name used by die for his New Caledonian species, necessitates 

 a new specific designation for Arber's type, which it is proposed 

 to rename Dadoxylon Arberi. From Liassic beds in Yorkshire 

 Miss Holden- has described a species as Araucarioxylon sp. which 

 she suggests may be one of the oldest representatives of the Arau- 

 carineae or perhaps a Jurassic example of Cordaites: the latter 

 identification is supported by a reference to the recorded occurrence 

 by Lignier of an Artisia^ in French Jurassic beds, a test of affinity 

 that cannot be accepted as satisfactory. 



There remains for consideration the debated question as to the 

 value to be attached to the occurrence of contiguous and flattened 

 pits as an index of Araucarian affinity when this feature is associated 

 with a type of medullary-ray pitting foreign to Dadoxylon. In 

 Gothan's genus Xenoxylon* the tracheids have usually large 

 flattened pits, but the pits on the radial walls of the medullary- 

 ray cells are very different from those characteristic of Arauca- 

 rineous wood. It is, moreover, not uncommon to find instances of 

 contiguous and alternate pits on the tracheids of a stem in which 

 the more usual type is the Abietineous arrangement. Gothan* 

 lays greater stress on the nature of the pitting on the walls of 

 medullary cells, but Jeffrey^ has discovered typical Abietineous 

 ray cells in the cone-axis of Araucaria and Agathis. Miss Holden' 

 goes so far as to maintain that the only feature which holds 

 absolutely is provided by the rims of Sanio : these are invariably 

 absent in Conifers with Araucarian affinities except on the first 

 few tracheids of the cone-axis of Araucaria and Agathis. This 

 author records as Araucarioxylon sp.^ a wood (described on a later 

 page) from New Jersey possessing opposite pits on some of the 

 tracheids, also rims of Sanio. We cannot lay down any definite 

 rules with regard to the sporadic variation in tracheal or medullary 

 pitting or as to the relative value to be assigned to one or other 

 character. The statement by Thomson^ that the ray cells of such 



1 Arber (0.5) B. p. 191, text-figs. 40—43. 



2 Holden, R. (13-) p. 540, PI. XL. fig. 28. ^ Sec p, 248, Vol. ni. 



" See page 248, Vol. ra. ^ Gothan (05). « Jeffrey (12). 



' Holden (14) (13=) p. 544. See also Sifton (15). 



8 Holden (14) p. 171. » Thomson (13). 



