During the last twenty years many single examples of vegetable forms from Carboniferous rocks have come into my possession, which were obviously different from any hitherto described. But I have carefully abstained from publishing such specimens until examples of each multiplied in my cabinet, enabling me to determine how far their apparently distinctive features were constant, and not merely individual, variations. Many such imperfectly known forms still occupy a drawer in my cabinet; but in the present Memoir I propose to describe several of which examples have accumulated so far as to enable me to speak with reasonable certainty as to their specific distinctiveness. In several of my previous memoirs I have from time to time called attention to a curious development of a medulla in the centre of the axial vascular bundle, especially of the Lepidodendra. This was especially done in the Memoir, Part III., when describing the Burntisland Lepidodendron , to which, as was also the case with the Arran form (Part X.), I have not yet ventured to give a specific name.