_Callixylon Trifilievi_ Zalessky. This genus is represented by a single species[771], 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_, _b_. 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 contiguous 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[772]. 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 with _Pitys_[773]. The above account is based in part on Zalessky’s description and partly on specimens in Dr Kidston’s possession. [Illustration: Fig. 489. _Callixylon Trifilievi._ A, C, the inner margin of the secondary xylem showing strands of primary xylem, _a_, _b_, and C. B, tracheids of the secondary xylem. (A, C, from photographs supplied by Prof. Zalessky; B, Cambridge Botany School.)]