only be made in the comparatively few cases in which the pith-cast is preserved. Its validity 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-cylinder 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[655]. 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[656] in 1877 described pieces of wood from the English Coal Measures and the Lower Carboniferous of Scotland which he referred to _Dadoxylon_ but without any specific name. These include the Coalbrookdale stem in which he had previously demonstrated the connexion between _Artisia_ and _Dadoxylon_. The structure of the xylem is like that in _D. Brandlingii_ and the specimens may belong to that species. The most interesting fact recorded by Williamson is the occurrence of double leaf-traces, a feature which led him to suspect a remote generic affinity to _Ginkgo_. This double trace may be an important diagnostic feature but unfortunately the majority of descriptions of species of _Dadoxylon_ throw no light on the character of the foliar bundles. Thomson and Allin[657] have recently pointed out that a double leaf-trace occurs in a stem from the Permian of Kansas described by Penhallow[658] as _Pityoxylon chasense_ and referred to that genus because of the supposed occurrence of resin-canals in some of the medullary rays: the canals are apparently leaf-traces traversing broad rays in the secondary wood.