438 STUDIES IN FOSSIL BOTANY 



for the leaf-bases, still attached to the stem, show in all 

 respects the typical Myeloxylon structure. The genus 

 Myeloxylon is therefore one of those which can now be 

 dispensed with, or at most be retained, as a matter of 

 convenience, for those petioles which have not yet been 

 referred to their particular species of Medullosa. 



The petioles of Medullosa anglica were of great 

 length, and branched repeatedly, the successive branches 

 diminishing in size, and undergoing some simplification 

 in structure. The whole evidently constituted the rachis 

 of a highly compound, probably bi- or tripinnate leaf. 

 The final ramifications, which are no more than a milli- 

 metre in diameter, contain only a very few vascular 

 bundles, but these still retain the same collateral, exarch 

 structure as those of the main petiole. Gum-canals 

 occur throughout the rachis ; the hypodermal structure 

 becomes simplified in the finer branches, consisting of 

 an almost continuous zone of peripheral sclerenchyma. 



Associated with the branched rachis, leaflets of 

 characteristic structure are found. As shown by trans- 

 verse sections, these leaflets are constantly revolute at 

 their margins (see Fig. 167, B); the parenchyma towards 

 the upper surface is of palisade-like structure, while the 

 lower portion of the mesophyll is more lacunar. The 

 lateral bundles, which traverse the lamina in an oblique 

 direction, are accompanied by fibrous elements. The 

 midrib is very prominent on the lower surface, and 

 shows the same structure as a small branch of the 

 rachis. From constant association and agreement in 

 structural details, there is no doubt that these pinnules 

 formed part of the Medullosa leaves. This conclusion 

 is of great interest, because the pinnules in question 



