250 COEDAITEAE [CH. 



the evidence clearly points to the Araucarineae the generic name 

 Araucarioxylon should be added in parentheses after Dadoxylon, 

 but whether or not this is done, a statement as to the geological 

 age of the fossil will in itself be some assistance in enabling the 

 student to form an opinion on the balance of probability in favour 

 of a Cordaitean or an Araucarian affinity. The course suggested 

 by Gothan^, namely to add Cordaites after Dadoxylon if an Artisia 

 pith is present, is rendered inoperative now that we know that a 

 discoid pith occurs in more than one genus. In this chapter we 

 are concerned primarily with Cordaites and with such stems as 

 may fairly be regarded as Cordaitean: examples of fossil wood 

 from later formations are dealt with in another place. A distinc- 

 tion between Araucarioxylon and Cordaioxylon stems has been 

 based by Fehx on the nature of the pith-casts; those of the 

 Artisia type he refers to Cordaioxylon, while Palaeozoic stems 

 with Tylodendron pith-casts are assigned to Araucarioxylon^. 

 This distinction can, however, only be made in the comparatively 

 few cases in which the pith-cast is preserved. Its vahdity is, 

 moreover, open to question. A Tylodendron (= Schizodendron) 

 cast shows on its surface the characters of the inner face of the 

 secondary xylem, projecting spindle-shaped areas representing 

 the inner ends of medullary rays and a reticulum of grooves 

 formed by the more resistant and prominent inner edges of the 

 rows of tracheids (fig. 746). A pith-cast of a stem in which the 

 destruction by decay of the medullary parenchyma had not 

 extended to the edge of the xylem-cyHnder might show transverse 

 diaphragms. The occurrence of Tylodendron casts means that 

 decay had extended to the surface of the wood. But in view of 

 the occurrence of Tylodendron casts in stems that are not those 

 of Cordaites a short account of the genus is given on another page*. 

 The main features of the stem of Cordaites have already been 

 enumerated. The stele agrees with that of Araucaria and Agathis 

 and especially with Agathis in the double nature of the leaf-trace. 

 Williamson* in 1877 described pieces of wood from the English 



1 Gothan (05) p. 15. = gterzel (03). 



^ See Chap. XLV. 



* WiUiamson (77) A. p. 226, Pis. vn.— ix. ; (80) A. p. 516, PI. xx. fig. 16 ; (83^) 

 A. p. 470. 



