_Colpoxylon aeduense_ Brongniart. The genus was founded by Brongniart[402] on a piece of stem 15 cm. in diameter from the Permian of the Autun district and regarded by him as a distinct type, with certain resemblances to recent Cycads. A thick section in the British Museum, 13 cm. in diameter (fig. 435, A), illustrates the main anatomical features described by Renault[403], to whom our knowledge of the genus is chiefly due. There are two large steles of irregular outline closely resembling those of _Medullosa Leuckarti_ (_cf._ fig. 416, H); each consists of a band of secondary xylem with broad medullary rays and a narrow zone of phloem enclosing a central region composed of parenchyma, in which strands of primary tracheids, both reticulate and spiral, pursue a more or less horizontal course, associated with a few small groups of vertical xylem-strands at the inner edge of the secondary wood. The manoxylic nature of the wood is clearly shown in fig. 436; the continuous ink-line marks the position of the cambium and the dots show the internal protoxylem. Homogeneous parenchyma surrounds the steles and beyond this is crushed tissue containing large secretory canals and nests of stereome fibres either as separate groups or in contact with the canals (fig. 435, C). In the same peripheral tissue occur scattered collateral vascular bundles (fig. 435, D) identical with those of _Myeloxylon_. The outer cortex of the stem is marked off from the more homogeneous inner region by a fairly distinct line where there is some indication of periderm. The anatomical features are clearly shown in fig. 436, a photograph from a section in Dr Kidston’s collection. At _a_ is an imperfectly preserved vascular bundle with a crescentic group of secondary xylem which is probably a leaf-trace that has just emerged from the secondary cylinder. Renault speaks of these more or less circular strands as possibly connected with reproductive shoots, but it is more probable that they are homologous with the strands in the pericycle and inner cortex of _Medullosa_ and represent leaf-traces before division into smaller collateral strands. Renault describes the stem as possessing seven vascular cylinders in the apical region and suggests branching of the main axis as the cause of the increase in number: there is, however, no evidence to support such correlation. The two steles seen in fig. 435, A, become merged at a lower level into a single stele of sinuous form (fig. 435, B). [Illustration: Fig. 435. _Colpoxylon aeduense_. A. Diagram of a transverse section of a specimen in the British Museum. (V. 9393. ½ nat. size.) B. Stele; much reduced from Renault’s figure. C, D. Secretory canal with fibres and collateral vascular bundle (Kidston Coll., 1946).] [Illustration: Fig. 436. _Colpoxylon aeduense_. Transverse section of half of the stem; _a_, stele of branch. Slightly enlarged. (Kidston Coll., 1946.)]