32G PROF. A. C. SEWARD AND MR. J. WALTON ON [vol. lxxix, 



seen to be compact, 1 with distinct growth-zones varying from 

 0*5 mm. to 9 mm. in breadth, and resembles D. lafoniense very 

 closely. The cross-sectional areas of the lumina of the spring- 

 tracheids may be as much as ten times that of the last formed 

 summer-elements which they succeed. This indicates well-marked 

 seasonal phases. The tracheids are pitted on the radial walls only. 

 The pits are bordered, and have a centric and generally circular 

 pore. The spring tracheid-pits may be quadriseriate, but the 

 biseriate arrangement is more usual (fig. 21, PL XXII). The pits 

 are as frequently opposite as alternate. On the spring tracheids 

 the pits are often in stellate groups (fig. 19, PI. XXII), a feature 

 reminiscent of Callixylon as illustrated by the species ft Oweni 

 Elkins & Wieland. 3 In this Upper Devonian type from Indiana 

 the pits occur in large groups, the pits in each group being 

 frequently opposite ; that is, on the same horizontal line on the 

 face of the tracheids. In the Falkland stem the pits on the 

 tracheids succeeding the larger spring-elements are often uni- 

 seriate and distant as in D. Pedroi Zeiller, 3 and in species of JRhexo- 

 xylon.^ The pits on the wider tracheids are frequently polygonal 

 when in contact, and the pore is occasionally elliptical and oblique. 

 Trabecule 5 have been observed passing radially through several 

 tracheids. 



The medullary rays (fig. 20, PI. XXII) are typically uniseriate, 

 although biseriate rays are not uncommon. The rays vary from 

 2 to 16 cells in depth : the cells are thin-walled, and span 2 to 4 

 tracheids. Owing to the thinness of the walls the ray-pits are 

 rarely preserved, and in radial sections one sees only the pits on 

 the underlying tracheids (fig. 19, PL XXII; text-fig. 4). In one 

 place, in the region of the summer-wood, pits on the ray-cells can 

 be seen (text-fig. 5) ; they cover about half the areas of one of 

 the bordered pits on the tracheid below the ray, and are -simple 

 and elliptical with the long axis radial. On some medullary 

 ray-cells larger pits are shown, which are probably of the nature of 

 ' eiporen '. 



The features seen in radial section agree with those of a piece of 

 secondary wood from Lafonia, referred provisionally by Halle to 

 Dadoxylon angustum Felix. The outstanding difference is the 

 extraordinary narrowness tangentially of the medullary ray-cells 

 (12-15 jx) in D. angustum. It may, however, be added that con- 

 siderable variation occurs in individual sections of some of our 

 specimens from Walker Creek and Fanny Cove : the breadth of 

 the ray-cells, as seen in tangential section, varies from a minimum 



1 Halle (11) cf. pi. ix, fig. 7. 



2 Elkins & Wieland (14). 



3 Zeiller (95) p. 623. 



4 Additional material of the South African genus Rhexoxylon Bancroft (13) 

 from the Middle Karroo formation recently examined by one of us [J. W.j 

 has revealed several new anatomical features ; a full account of the genus 

 will be published in the near future. [Phil. Trans. Roy. Soc. vol. ccxii (1923) 

 p. 79.] 



5 Seward (19) p. 135 ; fig. 6931 (p. 137). 



