238 CONIFERALES 





of stem showing 75 rings of growth. There are no resin-canals 

 and no xylem-parenchyma ; the tracheids have uniseriate bordered 

 pits on the radial walls only and they are almost invariably com- 

 pressed by mutual contact; the presence of a torus is a feature 

 characteristic of the Abietineae and not of the Araucarineae. 

 Bars of Sanio are present but there are no rims of Sanio. Tyloses 

 are abundant in the tracheids. The medullary rays are 2 20 

 cells deep, generally uniseriate though occasionally biseriate ; the 

 pitting is of the Abietineous type. In the abundance of tyloses 

 and in other characters the wood resembles Protocedroxylon 

 araucarioides Goth, a species transferred by Miss Holden to 

 Metacedroxylon, but it differs in the absence of tangential tracheal 

 pits and in the occurrence of biseriate medullary rays. 



Some fossil wood, which is not very well preserved, from 

 Middle Cretaceous rocks near lefren in the Gulf of Tripoli is de- 

 scribed by Negri as Protocedroxylon Paronai 1 . The tracheal pits 

 where biseriate are often alternate and compressed ; rims of Sanio 

 are absent: the latter feature, deduced from negative evidence, 

 and considering the state of preservation, is surely of little value. 



The presence of Araucarian pitting on the tracheids in several 

 Jurassic species is far from surprising in view of the prevalence 

 of that type of pitting in Palaeozoic stems; moreover an ad- 

 mixture of characters is a natural result of progressive develop- 

 ment. It is a matter of opinion with legard to the relative value 

 of tiacheal or medullary-ray pitting whether Protocedroxylon 

 should be placed nearer to the Araucarineae or to the Abietineae. 

 Miss Holden 2 discards the name Protocedroxylon for Metacedroxylon 

 on the ground that the former implies Abietineous affinity, a fine 

 shade of difference that hardly gives adequate expression to her 

 conclusion that 'Metacedroxylon araucarioides cannot be other 

 than an Araucarian Conifer.' 



XII. XENOXYLON. Gothan. 



Gothan 3 instituted this generic name for some Upper Jurassic 

 wood, originally described by Cramer 4 from Green Harbour, 



1 Negri (14) p. 340, PI. v. figs. 16; PL vi. figs. 13. 



2 Holden, R. (14) p 538 3 Gothan (05) p. 38. 

 4 Cramer (68) PL XL. ; Schroeter (80) p. 7. 



