The name _Medullosa_ was applied by Cotta[243] to three types, _Medullosa elegans_, _M. stellata_, and _M. porosa_, from the Rothliegende of the Chemnitz district. The first of these was recognised by Brongniart[244] as a distinct genus for which he proposed the designation _Myeloxylon_ and this was afterwards identified by Renault, Williamson, and other palaeobotanists as a petiole and not a stem. Further reference is made to _Myeloxylon_ on a later page. Cotta spoke of _Medullosa_ as the most puzzling of the genera dealt with in his ‘Dendrolithen,’ and in spite of the many additions to our knowledge the position of this Palaeozoic genus is still a fertile source of speculation. The generic designation _Medullosa_ is applied to stems, with or without petioles; petioles or rachises of fronds that frequently occur apart from stems are referred to the genus _Myeloxylon_. The leaves of _Medullosa_ include several well-known species of Carboniferous and Permian genera such as _Alethopteris_, _Neuropteris_ and others that have in recent years been transferred from the Filicales to the Pteridosperms. In a few instances seeds have been found in organic connexion with Medullosan foliage, and there can be no reasonable doubt that _Trigonocarpus_, some forms of _Rhabdocarpus_, _Pachytesta_, and other seeds represent the integumented megasporangia of _Medullosa_ or some closely allied genus. Before attempting to summarise the salient features of _Medullosa_ a description of a few selected types will serve to place us in a better position to consider the genus as a whole. The British species are placed first on the ground that they are both geologically the oldest though, historically, the most recently described, representatives of the genus; and in the organisation of the stem they are simpler than the continental species. Their resemblance to _Heterangium_ serves to some extent to bridge the gap between the majority of species of _Medullosa_ and the simpler types of Pteridosperms represented by _Heterangium_ and _Lyginopteris_. _Medullosa anglica_ Scott[245]. Prior to the discovery of this species the genus _Medullosa_ had not been recorded from Britain. A section in the Williamson collection recognised by Scott as that of a _Medullosa_ had been identified by Williamson as a large _Heterangium_ stem. An undescribed specimen was found by Arber[246] in the Binney collection at Cambridge which afforded some additional information as to the structure of the roots. The specimens on which Scott’s thorough description is based were obtained by Messrs Wilde and Lomax from the Lower Coal Measures of Lancashire. The stem of this oldest species has the habit of a tree-fern and is almost completely invested by the stout decurrent bases of the petioles of large spirally disposed compound fronds with a phyllotaxis of ⅖, the leaves of the same orthostichy being separated from one another by a vertical distance of approximately 10 cm.