60 DR. D. H. SCOTT ON A NEW TYPE OF MEDULLOSEZ 
only, limited to one side of the bundle, namely, in the case of the unilateral type, the 
side remote from the protoxylem (fig. 22). In the leaf-bases the vascular bundles are 
more scattered, and so a larger proportion of sclerenchyma per bundle may be necessary 
for mechanical efficiency. The distinction, however, is by no means an absolute one. 
In some cases the sclerenchyma is merely more strongly developed on the one side, 
where it may form a more or less continuous arc. 
One of the petioles, where the external tissues are unusually perfect, shows a distinct 
periderm, reaching 4 cells in thickness, and lying immediately below the small-celled 
epidermis, which, however, is only preserved here and there. The radial seriation is 
quite clear, and as the layer can be traced for a long distance, it was probably a normal 
formation rather than the result of a traumatic reaction. At some places the periderm- 
cells converge towards a slight depression of the surface, suggesting the presence of a 
lenticel. Lenticels are known to be formed on the petioles of Marattiacee *. The 
tissue below the periderm is ill-preserved, and no doubt consisted of thin-walled cells, 
which may probably have had an assimilatory function. 
The sclerotic zone of the hypoderma often has the same * Wyeloaylon Landriotii” 
structure as in the leaf-bases ; in some places, however, this zone increases in thickness, 
and the radial bands of sclerenchyma are elongated, approaching the type of 
M. radiatum t. 
Early in the present year I received from Mr. Peter Whalley, of Colne, Lancashire, 
several sections of petioles, which have proved to possess practically the same structure 
as the petioles of Sutcliffia insignis from Shore. Mr. Whalley describes the specimens 
as coming “from the Soapstone band, ‘from four to seven feet above the Mountain 
four-feet coal-seam (Lower Coal-Measures).” 
The sections are evidently from much smaller petioles than the Shore specimens, the 
diameter in one direction being as little as 8 mm., though, no doubt, somewhat reduced 
by compression. The specimens probably belong to the upper part of a petiole or to 
a branch of the rachis. They are well preserved, better than the Shore petioles, and all 
the features in the structure, as already described, are well shown. The hypoderma has 
radially elongated sclerotic bands, as is sometimes the case in the Shore petioles. The — 
outer portion of one of Mr. Whalley's specimens, including the hypoderma and three of 
the peripheral bundles, is shown in transverse section in Pl. 9. fig. 17. The agreement 
with the Shore specimens will be evident on comparison with fig. 16. I see no reason 
to doubt that the plants from these two sources are specifically identical. It is much to 
be hoped that Mr. Whalley may succeed in discovering a specimen of the stem also, - am 
from the Colne locality. 
An interesting question is that of the relation of Sutcliffia insignis to the petioles À E 
de scribed by Mr. Seward under the name Rachiopteris Williamsoni (Seward, 1894). 
The resemblance between the Shore petioles and Mr. Seward's fossil strongly impressed 
me from the first, and was at once recognized by Mr. Seward himself when I showed 
him the sections. 
* Potonié, 1881. T Renault, 1883, pl. 28. 
