i. _Leaves._ It has already been stated that in some cases petioles occur in organic connexion with Medullosan stems, notably in _M. anglica_ and _M. Leuckarti_: in the exceptionally rich collection in the Chemnitz Museum, which forms a fitting memorial of the work of the late Prof. Sterzel, there is a stem of _M. Leuckarti_ bearing large petioles of the type known as _Myeloxylon radiatum_. The occurrence of vascular bundles in the cortex of other species of stem identical with those in the attached petioles points to a uniform type of leaf-structure so far as regards the petioles and rachises of _Medullosa_. While it is clearly unnecessary to distinguish by a special generic title the petrified portions of fronds known to belong to certain species of stems, the frequent occurrence of detached petioles necessitates some distinctive term. The name employed is _Myeloxylon_: the genus was instituted by Brongniart in 1849 for Cotta’s species _Medullosa elegans_ the petiolar nature of which was suspected by Binney in 1872. _Myeloxylon_ (Medullosan leaf-stalks). 1832. _Medullosa elegans_ Cotta. 1865. _Stenzelia_ Goeppert. 1876. _Myelopteris_ Renault[279]; 1877, _Aulacopteris_ Grand’Eury[280]. There is a very close agreement in general anatomical structure between the numerous specimens of _Myeloxylon_ from the Permian strata of Saxony and France and the Coal Measures and Millstone Grit of England[281]; the genus is also recorded from the Upper Carboniferous of Kansas[282]. Two well-defined types instituted by Renault are, however, readily distinguished by the form of the hypodermal stereome strands. _Myeloxylon_ may be defined as follows: Oval or cylindrical branched axes, reaching a diameter of 15 cm., bearing pinnae having the characters of _Alethopteris_, _Neuropteris_, _Odontopteris_, and some other genera that were formerly classed as Ferns. Below a single-layered epidermis, in which stomata have been recognised, occur a few layers of parenchyma: this superficial tissue, which is rarely preserved, is succeeded by a hypodermal region consisting of parenchymatous tissue and numerous vertical groups of narrow thick-walled fibres arranged as radial plates or circular, oval, or reniform strands (the _Sparganum_ type of cortex). In the hypoderm as in the ground-tissue generally secretory canals, often accompanied by stereome, are a characteristic feature. The vascular system is represented by a considerable number of collateral bundles scattered through the ground-tissue and especially abundant in the outer region: the bundles sometimes assume a more or less regular disposition in concentric circles. Each bundle consists of a small group of xylem tracheids, for the most part spiral or scalariform, though reticulately pitted elements are by no means rare, with a single protoxylem group on the outer face next the phloem (fig. 420, B, C). As a rule the xylem is wholly centripetal, but occasionally the exarch structure becomes