A comparison of the stems described under the generic name _Medullosa_ reveals a considerable range in the grosser anatomical features superadded to certain fundamental characters denoting a common origin[414]. If additional data were available giving us a fuller knowledge of individual plants differences between species would be more clearly defined and would provide adequate grounds for the institution of new genera for some of the types now included in the comprehensive genus _Medullosa_. A proposal by Dr Lotsy[415] to adopt the names _Pecopteromedullosa_ and _Neuropteromedullosa_ rests primarily on the relatively unimportant difference between the fronds associated with certain Medullosan stems and, as Scott[416] points out, this two-fold division if applied to such a species as _Medullosa Leuckarti_ would result in its separation from species which anatomically are clearly of the same generic type. In _Medullosa anglica_, one of the oldest and simplest types, there are three steles of equal importance, and each of them is practically identical with the single stele of _Heterangium_. Each stele—as indeed all Medullosan steles—consists of a strand of primary xylem enclosed by secondary xylem and phloem, and it is obvious that the development of a constantly increasing cylinder of secondary conducting tissue about three centres would lead to serious mechanical difficulty: a stem constructed on the plan of _Medullosa anglica_ or the smaller _M. pusilla_ could not increase the thickness of its secondary vascular tissue beyond a certain point without detriment to its efficiency. In some types this difficulty is partially overcome by the production of complete concentric cylinders of centrifugally developed conducting tissue external to an inner system of concentric steles agreeing individually with those of _Heterangium_ (fig. 415, B). _Medullosa anglica_, regarded from the point of view of the architectural efficiency of its vascular system, affords a much less promising _point d’appui_ for further evolution than some of the forms described under _Medullosa stellata_ in which the mechanical impasse is avoided by the adoption of the cycadean plan as represented by such genera as _Cycas_ and _Macrozamia_. The English species _Medullosa centrofilis_ (fig. 417) affords the first example of a characteristic Medullosan feature, namely the presence of a small concentric stele in the central region of the stem: this so-called star-ring differs not only in its smaller dimensions but in its more cylindrical form from the larger peripheral steles. In the later Permian species, _e.g._ _Medullosa porosa_ and _M. Solmsi_, the single star-ring of the older _M. centrofilis_ is replaced by a large number of precisely similar conducting strands. These star-rings are structurally comparable with the cortical steles of _Cycas_ and, in position, with the medullary system of bundles in a _Macrozamia_; they are essentially cauline and take no part in the