XXXIV] CALLIXYLON 291 



Callixylon. Zalessky. 



Callixylon Trifilievi Zalessky. This genus is represented by 

 a single species 1 , originally referred to Dadoxylon, founded on 

 material from Upper Devonian rocks in the Donetz basin, Russia, 

 and based solely on the anatomical characters of the secondary 

 wood and central region of an arborescent stem. The wood is 

 pycnoxylic and of the Araucarian type ; the inner portion of the 

 xylem-cylinder is divided into groups, similar to the regular 

 wedges in a Calamite stem, consisting of tracheids converging 

 towards an obtuse apex occupied by a primary xylem strand, 

 fig. 489, A, a, 6. On the radial walls of the tracheids the bordered 

 pits not infrequently form a single row of flattened ovals, but more 

 usually there are two or sometimes three rows of alternate con- 

 tiguous pits (fig. 489, B) : circular and separate pits also occur. 

 As Zalessky states, the pits do not always occupy the whole 

 radial face ; unpitted patches sometimes interrupt the continuity 

 of the pitted areas 2 . Similar circular and more scattered pits 

 are unusually abundant on the tangential walls. There are no 

 complete rings of growth. The medullary rays are narrow and, 

 except at their dilated inner ends, uniseriate; usually one or a 

 few cells deep, they may reach a depth of 12 or more cells. The 

 pits on the radial walls of the ray-cells are said to be 4 7 in the 

 field. The pith consists of thin-walled flattened parenchyma 

 frequently elongated in a radial direction. At the inner edge of 

 the secondary xylem and generally in contact with it are several 

 anastomosing strands of primary xylem, mesarch in structure 

 but with the protoxylem nearer the inner edge. These bundles 

 may be single (fig. 489, A, a) or double (fig. 489, A, b, and C) 

 and closely resemble those of Pitys antiqua except in their closer 

 relation to the secondary wood. The leaf-traces have not been 

 described, but the occurrence of twin-bundles like those in fig. 

 489, C, suggests that they were double. The primary-xylem 

 elements show particularly well transitional forms of pitting 

 connecting the multiseriate and scalariform types. 



Callixylon, though conveniently and justly regarded as a distinct 

 genus, exhibits in its primary xylem a fairly close agreement 



1 Zalessky (09); (11) p. 29, PI. iv. 



2 Zalessky (11) PL iv. fig. 3. 



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