MEDULLOSEAE 44* 



of a number of the principal leaf-traces springing from 

 the steles. The principal leaf-traces were concentric in 

 structure, and surrounded by secondary wood and bast, 

 but, as the bundle underwent subdivision, its branches 

 gradually lost their secondary tissues, and, at the same 

 time, assumed a collateral, exarch structure. The 

 petiole and rachis had in all respects the structure of 

 " Myeloxylon Landriotii." The stem bore triarch, 

 adventitious roots, forming secondary wood and bast, 

 and coated by a periderm of pericyclic origin. 



The form of Medullosa just described, which happens 

 to be the one most completely known up to the present 

 time, represents the genus, as it were, in its simplest 

 expression. 1 Several Continental species had previously 

 been investigated, all of which are of later geological 

 age than M. anglica, belonging chiefly to the Permian 

 beds. They all have a more complex vascular system 

 than the English species ; the steles are usually more 

 numerous, and in all cases show a differentiation among 

 themselves. The larger steles are associated to form a 

 single or double peripheral ring, while others, of much 

 smaller size, are grouped in the central region of the 

 stem. In most of the specimens, as, for example, in 

 those of the form known as Medullosa stellata, the 

 peripheral steles are few in number, and much elongated 

 in the tangential direction, so that they collectively 



1 A very small Medullosa (named provisionally M. pusilla), the stem 

 with the leaf-bases not exceeding 2 cm. in diameter, has since been found 

 by Mr. P. Whalley of Colne, Lanes. The stem has three steles and agrees 

 very closely with AL anglica except in size. In the Renault collection at 

 Paris there are sections, under the name Heteranghim geriense, of the 

 stem of a Medullosa, apparently of the same type as M. anglica, from the 

 Upper Coal-measures of the Ride de Gier, near St. litienne. This species 

 may bear the name Medullosa geriensis. 



