_Antarcticoxylon Priestleyi_ Seward. The specimen on which this genus was founded was discovered by Mr Raymond Priestley[785] in a boulder, probably derived from the Upper Beacon sandstone, on the Priestley Glacier south of 74° S. lat. in the course of the journey of the Northern Party of Capt. Scott’s second Antarctic Expedition. The type-specimen is a piece of silicified stem 1 ft long and 3 ins. in diameter; there were no tissues preserved external to the secondary wood and it is impossible to say what proportion of the original thickness is represented. Annual rings are clearly marked at least macroscopically (fig. 491, C) though there is very little difference between the spring and summer tracheids: the centre of the compressed stem is occupied by a lighter coloured elliptical area 7 × 2 mm. which superficially resembles a pith, but in the peripheral region it consists of portions of a cylinder of spiral and scalariform tracheids, the actual pith being not more than 2–3 mm. in breadth. The pith consists of lacunar parenchyma separated by horizontal bands of dark cells containing some secreted substance (fig. 491, F): the preservation is, however, not sufficiently good to enable one to describe it in detail. The secondary xylem is of the pycnoxylic type; the tracheids have either a single row of contiguous and partially flattened pits on their radial walls or a double row of alternate polygonal pits; the medullary rays are nearly always uniseriate (fig. 491, E) and from 1 to 24 cells in depth. At the inner edge of the secondary wood there was a fairly broad zone of more delicate tracheids (fig. 491, A, _x_) characterised by spiral or scalariform bands and by their relatively small diameter. This innermost zone, which supplies the leaf-traces, is spoken of as the primary xylem; it appears to be endarch though this cannot be definitely determined. A characteristic feature of the primary xylem in the perimedullary region, as also in the leaf-traces on their way through the secondary wood, is the presence of short and broad tracheids (fig. 491, D, _t_) with reticulate thickening bands: these short elements may represent centripetal xylem and are similar to the short tracheids described by Scott[786] in the sheath of _Mesoxylon Lomaxi_ and to the larger elements in the stem of _Megaloxylon_[787].