mesarch by the occurrence of a few centrifugal tracheids. The phloem, rarely preserved (fig. 420, B), consists of narrow sieve-tubes with parenchyma, and the bundle as a whole is often partially enclosed by a sheath of fibres. Superficially the anatomical structure is similar to that of the petioles of _Angiopteris_ or _Marattia_, and both Williamson[283] and Renault placed _Myeloxylon_ in the Ferns; but the collateral form of the vascular bundles, the position of the protoxylem, and the arrangement of the hypoderm tissues, are cycadean features. _Myeloxylon radiatum_ (Renault). [Illustration: Fig. 420. A, B, _Myeloxylon radiatum_; A, petiole ⅔ nat. size; B, vascular bundle. C, _Myeloxylon_ sp. vascular bundle; _px_, protoxylem. D, _Medullosa anglica_; section of pinnule. (A, B, after Zeiller; D, after Scott.)] This type is characterised by the radially elongated stereome of the hypoderm. Prof. Zeiller[284], who has given a very clear and concise description of _Myeloxylon_, is disposed to regard Cotta’s _Medullosa elegans_ as a specifically distinct form on the ground that there are two concentric zones of stereome in the hypoderm; but this feature is shown only in one of Cotta’s figures, and Weber and Sterzel[285] point out that a doubting of the hypoderm zone may be caused by accidental juxtaposition of two faulted pieces of peripheral tissue. The drawing reproduced in fig. 420, A, shows the structural plan of an unusually large petiole from the Permian of Autun: a portion of the outer tissue is seen in fig. 418, A. The vascular bundle, fig. 420, C, from a Millstone Grit specimen[286], shows the centripetal nature of the xylem and fragments of phloem in the outer half of the bundle, with imperfectly preserved fibres abutting on the xylem. The characteristic hypoderm is shown also in fig. 418, A; the double xylem strand on the left illustrates a common feature caused by the branching of vascular bundles. Several secretory canals are scattered in the ground-tissue. The pinnules of _Myeloxylon radiatum_, or at least of some specimens, have been shown by Renault to be of the _Neuropteris_ type. _Myeloxylon Landrioti_ (Renault)[287]. In this species the distinguishing feature is the occurrence of the hypodermal stereome in the form of circular, oval, or reniform strands in place of the radial plates of _M. radiatum_. It is this form of petiole that was borne by the stems of _Medullosa anglica_ and _M. Leuckarti_. In _M. anglica_ the pinnules (fig. 420, D) are of the _Alethopteris_ type, almost certainly _A. lonchitica_. Renault and Zeiller have described French specimens of _Myeloxylon Landrioti_ bearing pinnules like those of _Alethopteris aquilina_ and _A. Grandini_. _Myeloxylon topekense_ (Penhallow).