Sahni.—On the Branching of the Zygopteridean Leaf. 377 
The two sections possibly diverged from a hypothetical Devonian 
species of the genus (see Table, Fig. 3) which had circular loops. Kidston 
and Gwynne-Vaughan ( 9 ) have already commented upon the tendency in 
some Zygopterideae to extend their peripheral loops in the antero-posterior 
direction. The four Upper Carboniferous species of Clepsydropsis^ C. 
{Ankyropteris) Williamsoni , corrugata , Grayi , westphaliensis , belonging to 
the section Ankyropteris , are named in order according to the degree of 
this extension. 
C. westphaliensis would seem to be the most specialized ; its peculiar 
mode of pinna-trace formation, in which process the ‘ filament ’ is reinforced 
by the c antenna is probably the result of the feebleness of the * filament * 
to nip off a closed ring on its own account. 
I am indebted to Dr. Scott for criticism on another point which I had 
omitted to discuss for considerations of space. As is well known, the pinna- 
trace in the genera Metaclepsydropsis, Diplolabis , Stauropteris , and Dineuron 
comes off as a single piece, while in Etapteris it arises in two distinct pieces 
which soon unite. It might at first seem doubtful, as Dr. Scott pointed out 
to me, that there is in Etapteris also a single row of pinna-traces on each 
side of the leaf-trace. This objection is adequately met by saying, as 
Dr. Gordon has already done (6, p. 186), that Etapteris has a more highly 
specialized form of pinna-trace origin than the other four genera, with 
which (see above, p. 370, foot-note) probably Zygopieris primaria will also 
have to be reckoned. In this respect Etapteris would thus stand towards 
the other genera of Zygopterideae as many higher Ferns with a double 
leaf-trace stand towards others with simple traces. 
Incidentally, it is tempting to follow up the above analogy a little 
farther. The tissue filling up the peripheral loops of the leaf-trace, and 
also that enclosed by the pinna- and aphlebia-traces, is in origin a portion 
of, or at least homologous with, the ‘ mixed pith * or internal xylem of the 
stem, while the rest of the trace is its external xylem. Professor Lang (10, 
p. 239) has already compared the outer and inner xylems of the Ophioglos- 
saceae to the similarly named tissues in the Zygopterideae. The Clepsydro- 
psis type of leaf-trace and pinna-trace, which does not cause a gap in the 
external xylem, may thus stand to the condition seen in Diplolabis , &c. 
(where a gap is caused, for the loop 1 opens ’), in the same relation as 
corresponding types of leaf-trace origin observed by Professor Lang in 
Botrychium Lunaria ( 10 , p. 237). The gapless condition would thus by 
analogy indicate a lower organization than that in which the continuity of 
the external xylem is disturbed. 
In fact, this is another important character upon the basis of which the 
Zygopterid petioles may be divided into the two groups referred to on 
p. 374. These two groups may conveniently be called, after their most 
primitive genera— 
