—  #£ON THE STRUCTURE AND AFFINITIES OF METACLEPSYDROPSIS DUPLEX. 187 
Of course all this grouping has been based on the hypothesis that the forms of the 
_ trace at low levels in the petiole (7.e. at early stages of development) are comparable 
with ancestral forms. This is also the basis of the work of Kipsron and GwyNNE- 
Vaucuan on the fossil Osmundacez, and of Sinnort’s studies on recent ferns. The 
hypothesis seems reasonable, and when applied to Diplolabis and Metaclepsydropsis 
(two forms closely allied on other grounds) the results lead to the same conclusion, 
and show that evolution was parallel in these two genera. 
settee Rin ee 
As far as the relationships between the Osmundaceze and Zygopteridex are con- 
cerned, the stem of M. duplex occupies an important position. In the medullation 
of the Zygopterid stele the series taken was Diplolalbis, Ankyropteris corrugata, 
Etapteris di-upsilon (Z. Gray). Now this is open to criticism, for the first and last 
named have quadriseriate primary pinne and petiole-traces with two planes of 
Profoclepsydropsis 
ae 
Dineuron 
Culrn 
Clepsydropsis 
Mefaclepsydrop sis Efapreris 
Lower 
Carboniferous 
Liplolabis 
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Coal 
Measures 
Permian 
Zygopleris 
y Text- Fic. 5.—Table to show the relation of Metaclepsydropsis to the other Zygopteridee which have quadriseriate pinne. 
Sy. metry, while the central type has biseriate primary pinne and one plane of 
metry in the petiole-trace. If we substitute M. duplex for Ankyropteris we 
ngthen the position from two points of view, for (1) we get a series of forms which are 
lar in all respects, and (2) we may construct a parallel series in the Zygopteridez with 
rlate primary pinne and one plane of symmetry, thus :—Ankyropteris corrugata— 
|. scandens. 'To complete the series in this second main division of the Zygopteridez 
e only want a form with a solid stem stele similar to Diplolabis. Clepsydropsis may 
ly this form, for the stems referred to that genus by Dr Paut Berrranp”® are not 
ove suspicion. Typical Clepsydropsts petioles have not been traced into these stems, 
and until this is done the Cladoxylons cannot be accepted as the stems of Clepsydropsis. 
Meanwhile the general trend of evolution as shown in the Zygopteridese is parallel 
to that demonstrated by Kipsron and Gwynne-VaucuHan in the Osmundacez, namely, 
from a simple to a more complex type, 7.c. both groups show an ascending series as 
we pass upwards in the geological succession. 
.. * Comptes Rendus des Séances de V Académie des Sciences, Paris, 16th November 1908. 
TRANS. ROY. SOC. EDIN., VOL. XLVIII. PART I. (NO. 8). 29 
