100 DR, D. H. SCOTT ON THE HETERANGIUMS 
Autun (H. punctatum *, H. Renaultii*, H. Duchartrei *, and H. bibractense) 
are referable to the subgenus Polyangium. 
9. The Lower Carboniferous species /7. Grievii may be taken as the type 
of another subgenus, u-heterangium, characterized by the leaf-trace con- 
sisting of a single bundle only, where it leaves the stele, this bundle either 
remaining undivided on its outward course or dividing into two strands only. 
One Dulesgate species, H. minimum, falls under this subgenus, as do also 
four of the Ostrau species described by Dr. Kubart, namely H. alatum, 
+. polystichum, H. Schusteri, and H. Sturii. The peripheral xylem-strands 
are, on the whole, less distinct in Hu-heterangium than in Polyangium, but the 
degree of their differentiation varies in the different species. The xylem- 
strands may either be “centrally mesarch” or show a tendency towards 
exarchy. A third subgenus may be required for the Lyginopteroid species, 
such as M. Andrei, discovered by Dr. Kubart. 
6. The polydesmie species of Heterangium (Polyangium) show an interesting 
analogy with Medulloseæ and with the protostelic Calamopityee. There is, 
however, no sufficient evidence of any direct phylogenetie relation to either 
group. With Rhetinangium there is no special affinity, for, while this genus 
appears to have much in common with /eterangium, its leaf-trace is complex 
and not multiple, thus presenting only a remote analogy with that of the 
polydesmie Heterangiums. Megaloaylon is altogether more remote. 
The generic characters given on p. 59 of this paper can now be made more 
precise as regards the nature of the leaf-trace in different subdivisions of the 
genus as stated in the Summary above, under the headings 2 and 5. 1t also 
appears that in such species as /7. alatum, Kubart, and H. polystichum, Kubart, 
the leaves were small and probably very different from the foliage of Spheno- 
pteris elegans, attributed with good reason to H. G'rievii. 
I am much indebted to my friends Prof. F. W. Oliver, F.R.S., and Prof. 
F. E. Weiss, F.R.S., for the loan of slides in the University College, London, 
and the Manchester Museum Collections. Without the opportunity of con- 
sulting these preparations, in addition to those in the Williamson Collection 
and in my own, it would have been impossible to deal at all adequately with 
the subject. 
The majority of the sections described were eut by Mr. Lomax, to whom 
the discovery of //. shorense and of the Dulesgate species is due. 
The photographie illustrations are the work of Mr. W. Tams, while the 
drawings were made by Mr. G. T. Gwilliam, to both of whom my best thanks 
are due for their valuable aid. 
* These three may perhaps be forms of the same species. 
T I 
