Anatomically the main features of the stelar system of _Medullosa_, neglecting the secondary xylem and phloem, are in closer agreement with the stems of Ferns than with those of any other plants. It has been shown that the genus _Heterangium_ bears a close resemblance to _Gleichenia_ in the structure of the primary stele (fig. 418, C): one of the oldest types of _Medullosa_, _M. anglica_, may be described as a _Heterangium_ with three steles and may be compared with a dictyostelic Fern in which the irregular vascular framework is made up of three main strands. In certain types of _Medullosa_ (fig. 416) the ground-plan of the vascular system recalls that of a solenostelic Fern, while in others the greater complexity suggests comparison with such Ferns as _Matonia_, _Angiopteris_, _Psaronius_, or _Cyathea_; ‘it is as though Nature were at the Carboniferous moment in the midst of a series of amazing engineering experiments, most of which were either buried deep in Palaeozoic oblivion, or permitted to survive only as vestigial relics and atavistic ghosts’[240]. Though many Medullosae resemble Ferns there is an important difference between the two groups in the origin of the various plans of Medullosan stelar systems: in Ferns the leaf is the determining factor in the evolution of stelar arrangement, while in _Medullosa_ the occasional interruption of a solenostele or the development of an apparently complex dictyostele are features independent of the leaf and leaf-traces. In the structure of the secondary xylem and phloem and in root-structure _Medullosa_ agrees with recent Cycads. The genus is in short a generalised type with filicinean and cycadean affinities. In the possession of seeds borne on modified pinnae of compound fronds, _Medullosa_ resembles both _Cycas_ and the Lyginopterideae. The seeds exhibit a fairly close agreement with those of _Lyginopteris_, _Heterangium_ and recent Cycads, but they appear to have advanced further towards the cycadean type than is the case with the closely related seeds of the Lyginopterideae. The microsporophylls are very imperfectly known but they were undoubtedly much less advanced and more fern-like than the megasporophylls. The genus _Medullosa_ is recorded from the Permian strata of Saxony, France, and Bohemia[241]; also from the Coal Measures of England, and the discovery of petrified petioles of _Myeloxylon_, the type borne on Medullosan stems in European species, may be taken as evidence of the existence of the genus in North America during the Carboniferous period[242].