26 i 
Medullosa from the Lower Coal Measures. 
difference in size in the three species there appears to be a difference in the 
development of the hypodermal layers of the leaf-bases. 
Very numerous leaf-trace strands are scattered throughout the ground 
tissue of the leaf-bases ; they appear to be orientated in no definite manner, 
and branching and anastomosing of the bundles occasionally occurred 
(PI. I, Fig. 3). 
Each leaf-trace strand is collateral in type ; the phloem is almost 
invariably represented by a space, on which the exarch protoxylem abutted 
(Text-figs. 4 and 5). The xylem consists of spirally thickened tracheides; 
it is surrounded by a single layer of thick-walled fibres, as in M. pusilla . 1 
Many of the leaf-trace strands are surrounded by a sheath of well- 
preserved cells, differing somewhat in appearance from the cells of the 
ground tissue ; the cells of this sheath show in places a regular seriation of 
the elements, and in many cambial divisions are taking place. (Cf. s. in 
Text-fig. 4 with in PI. I, Fig. 3.) 
Speaking generally, the hypoderma, the vascular strands, and the gum 
canals are so closely similar in the new specimen to those in Medullosa 
anglica , that except for the difference in size it would be difficult to 
distinguish between the leaf-bases of the two fossils. 
VII. Affinities. 
With regard to the affinities of the new specimen there is little to say, 
for the general organization of the stem, with its adherent leaf-bases entirely 
clothing the surface ; the close similarity of the leaf-base, both in general 
structure and in histological details, with that of Medullosa anglica and 
M. pusilla ; and finally, the agreement in practically every detail between 
the steles of the specimen and those of M. anglica and M. pusilla , leave no 
doubt as to its very close relationship with those fossils. 
There are, however, certain features which serve to distinguish the 
specimen from these species. The size of the stem is distinctly smaller 
than that of any described specimen of M. anglica , and it appears to be 
intermediate between that species and M. pusilla : the dimensions given by 
Scott 2 for M. anglica are 10-5 x 3.7 cm., the present specimen as nearly as 
can be estimated measures 5 x 1*5 cm., while M. pusilla is 2.2 x1.3 cm. 
Little importance would be attached to such a character considered alone, 
but, taken in conjunction with the following differences, it may possibly be 
of some diagnostic value. 
The arrangement of the leaves could not be definitely determined, but 
it does not appear to be consistent with the two-fifths phyllotaxis of 
Medullosa anglica ; 3 the stem in M. pusilla appears to have been covered 
1 M. pusilla , loc. cit. , p. 225. 2 M. pusilla, loc. cit., pp. 221-2. 
3 It is possible that this may be a local peculiarity, and that the phyllotaxis may prove to be 
two-fifths when further specimens are available for examination ; such a local variation in the leaf 
arrangement is not uncommon among living plants. 
T 
