a oe 
ON THE STRUCTURE AND AFFINITIES OF METACLEPSYDROPSIS DUPLEX. 181 
larity ; it is a dichotomy in each case, and in neither case has any “axillary” branching 
been recorded. It is also worthy of notice that the Botrychioxylon-like specimen shown 
in Pl. IV. fig. 46 is paralleled in one of Williamson’s figures of A. corrugata.* 
The petiolar development, however, is entirely different in the two species, A. cor- 
rugata having only two rows of primary pinnz, and these, as Kipston and GwyNNE- 
VAUGHAN point out,t being little more than scale leaves. 
The French Permian species A. Brongniarti is also very similar to M. duplex as far 
as its stem-stele is concerned. In size they are almost identical, but in A. Brongmarti 
the radiating arms are exceedingly well developed and the tracheides have all scalariform 
thickenings on their walls. Very little is known about this species, particularly about 
the petiole-trace, but the petiole referred by Renavr to the species is distinctly of the 
Ankyropteris type. The branching of the stem is of the “axillary” type now shown to 
be of the nature of an unequal dichotomy. 
Compared with Htapteris di-upsilon (Zygopteris Gray), the stem of M. duplex is 
very distinct. he stellate structure of the stem xylem in the former species is the 
result of the rapid emission of petiole-traces, and, when one has departed, the outline 
becomes rounded and not stellate at that part of the stem. The arms of parenchyma 
and tracheides radiating from the central zone of the stem xylem are much more 
prominent here than in A. corrugata or A. Brongniart; and the type of branching is 
an unequal dichotomy. ‘The petiole-trace which is emitted from the stem belongs to 
the third group, as defined by Ktpsron and Gwynne-Vaucuan. This correlation of the 
petiole known as Htapteris di-wpsilon (Williamson) with Zygopteris Grayi has only 
recently been published by Dr Kinston.{ The species is therefore distinct from 
M. duplex in all its salient features. 
Ankyropteris (Zygopteris scandens) (Stenzel) is another Zygopterid fern with a 
stellate stem-stele. It was in this species that the presence of the “axillary” branch 
was first demonstrated, and in some respects it is closely similar to Htapterts di-upsilon. 
The petiole-trace, however, is of the Ankyropteris type. It is thus quite distinct from 
M. duplex. 
Quite apart, then, from the appearance of the petiole-trace, M. duplex may be dis- 
tinguished from all other species of the Zygopteridez by the structure of the stem. As 
regards the petiole-trace itself, there is no species so far described with which it is likely 
to be confounded in its mature stages ; but, as has already been shown, the early stages of 
the petiole might quite well be confused with Dinewron. It is true that no specimen 
of Dineuron has been discovered which has as large a stele as M. duplex, but it is con- 
ceivable that some species of Dineuron may be discovered with a petiole-trace as large 
as that of the early stages in the trace of M. duplex. None of the early stages of 
petiolar development in the latter species is comparable with Clepsydropsis. In 
* Wiiuiamson, Phil. Trans. Roy. Soc., vol. clxvii., 1876, pl. v. fig. 19. 
+ Kipsron and Gwynne-Vaua@uan, Trans. Roy. Soc. Hdin., vol. xlvii., 1910, 
£ Ann. Bot., vol. xxiv., April 1910. 
