OF THE BRITISH COAL MEASURES. 9T 
A few words may now be added about other genera which present points 
of comparison with Heterangium. The nearest of these is doubtless Rhetin- 
angium, founded by Dr. Gordon on the species R. Arberi, which he discovered 
in the Pettyeur beds of the Caleiferous Sandstone Series (Gordon, 1912). 
The plant is thus of Lower Carboniferous age and contemporary with Meter- 
angium Grievit. The structure is protostelic, and the general appearance 
of the transverse section suggests a rather large stem of Heterangium. 
The new genus, however, differs from Heterangium in important characters : 
there are no sclerotic plates in the cortex or perieycle; the primary xylem- 
strands are exarch, not mesarch, and the leaf-traee is a complex corrugated 
body with numerous external protoxylem groups. Minor peculiarities are 
the great development of secretory sacs and cells (also present in some species 
of Heterangium) and the enormous enlargement of the leal-base, which 
almost equals the whole stem in thickness. The Sparganum, or more strictly 
Dictyoaylon, outer cortex is very finely developed, as in Medullosa. The 
exarch character of the xylem-strands is important; we have no evidence 
that true exarchy occurs in Heterangium, though, as we have seen, there are 
considerable variations in the degree of mesarehy. On the other hand, the 
structure of the metaxylem seems to me to be altogether that of a Heterangium, 
of the type, like H. shorense or H. tiliwoides, where the tracheides are in 
definite packets, separated by a reticulum of parenchyma. The secondary 
wood is of the ordinary Heterangium kind, with somewhat large rays opposite 
the spaces between the peripheral xylem-strands. 
But the most interesting point for our comparison is the leaf-trace, a 
remarkable and unique structure. It embraces several of the peripheral 
xylem-strands, all passing out together and all in connection laterally with 
each other. The connection being on the adaxial side, the leaf-trace consists 
of a number of fused U-shaped bodies, concave outwards. The protoxylem- 
groups, on the extreme outside, are numerous—six or more in number. 
The massive trace may remind us for a moment of a meristele of Suteliffia 
(Scott, 1906), but is really quite different, for it never breaks up or divides in 
any way, but passes out unaltered into the petiole. It is a complex but 
never a multiple leaf-trace. — Zhetinangium thus also differs fundamentally 
from our polydesmie Heterangiums, where the trace from its origin consists 
of two distinct and simple bundles, which divide further on their outward 
course to form more bundles of like nature. Rhetinangium is a most inter- 
esting parallel development, doubly interesting from its antiquity, bui it has, 
in my opinion, no special affinity with the polydesmic species of Heterangium. 
Prof. Seward’s genus Megulowylon (Seward, 1899) is more remote from 
Heterangium. The large stem agrees with MRhetinangium in the exarch 
strueture and in the presence of several protoxylem-groups on the abaxial 
side of the leaf-trace, but is peculiar in the character of the metaxylem, 
which consists for the most part of short wide tracheides, apparently adapted 
