_i. Stems._ The stems assigned to _Cladoxylon_ are characterised by a complex system of steles, either simple or branched and occasionally anastomosing, presenting in transverse section the form of oval or cylindrical strands or narrow, straight or curved bands arranged on a more or less clearly marked radial plan (fig. 459, A, B, D). In some stems the primary vascular tissue is enclosed by secondary xylem and phloem (fig. 460, B), while in others (Unger’s _Arctopodium_, _Hierogramma_[513], _Syncardia_) there is no evidence of secondary thickening. The diagrammatic drawing represented in fig. 459, F, shows a section of a small axis, regarded by Unger and Solms as a petiole (3 mm. in diameter), containing four vascular strands composed exclusively of primary xylem, each with one or, in the case of a double strand, two protoxylem groups. This type may be a slender stem or branch or possibly a petiole. The other extreme, as regards complexity of vascular structure, is represented by such stems as those shown in fig. 459, A, B, D. In Unger’s _Cladoxylon mirabile_ (fig. 459, A; fig. 460, B) the stem reaches a diameter of 3 cm. and consists of several radially disposed plates of vascular tissue with an occasional smaller oval or cylindrical stele embedded in a ground-tissue composed of thick-walled cells. The plates are curved like a UU or sinuous and not infrequently anastomosing. In a section of this type figured by Unger the vascular plates appear to form a complex anastomosing system, but Solms[514] states that the drawing exaggerates the amount of fusion between the strands, and an examination of a section in the collection of the English Geological Survey cut from Unger’s specimen enables me to confirm this statement. Each vascular plate consists of a narrow median region composed of primary tracheids with a scalariform type of pitting surrounded by secondary tracheids with interspersed medullary rays one cell broad. The thickness of the secondary xylem varies considerably in the same specimen and in places this tissue is hardly represented, a fact of importance in view of the very striking resemblance between _Arctopodium_ and _Cladoxylon_, the sections referred by Unger to the former genus having steles without any secondary xylem. The occurrence of one or two elongated spaces (shown in black in fig. 459, A) near the distal end of each plate mark the position of the protoxylem tracheids. Fig. 459, C, represents a stele of a stem referred by Unger and by Solms to _Cladoxylon dubium_ which shows the typical _Cladoxylon_ structure, namely the central primary xylem with distally placed protoxylem and the enclosing sheath of secondary xylem. In the stem shown in fig. 459, D (_C. dubium_) there are 12 steles, each constructed on the plan already described, differing in their relatively broader and shorter form and in the greater breadth of the secondary xylem from those seen in fig. 459, A (_C. mirabile_). The black areas in fig.