ON THE STRUCTURE AND AFFINITIES OF METACLEPSYDROPSIS DUPLEX. 183 
Arms of inner zone radiate into outer zone in certain places. Outer xylem of long, 
broad, reticulate tracheides. Protoxylem groups, of scalariform elements, not cauline 
but decurrent from the petiole into the stem, disappear at outer margin of inner xylem 
zone. Cortex without sclerenchyma. 
Petiole-trace at first elliptical in transverse section, with a sunk protoxylem group 
near each end. Ends ultimately enlarged and trace becomes hourglass-shaped, with 
sinus (groove) at the middle of each end. Protoxylem groups, two at each end, situated 
on sides of sinus. Tracheides with reticulate thickenings, except protoxylem elements, 
which are scalariform. Primary pinne in four orthostichies; inserted on petiole in 
pairs. Tertiary pinna-traces join secondary pinna-traces in two orthostichies. 
Insertion of secondary pinne on primary similar to tertiary on secondary. ‘Trace of 
primary pinna sometimes closed, but open lower down. Last traces to join primary 
pinna-traces are from aphlebie. Aphlebia-traces bifurcate during passage through 
petiole-cortex, and both branches pass out at the same level. 
Pinna- and aphlebia-traces unite to form pinna-trace-bar, which joins petiole-trace. 
Root-traces large, barrel-shaped, diarch. 
Foliage and sporangia unknown. 
Localities.—Calciferous Sandstone Series (=Culm), Pettycur, Fife; Culm of 
Régny.* 
CONCLUSIONS AND GENERAL CONSIDERATIONS. 
The discovery of stems of Zygopterid and Osmundaceous aftinity has recently 
thrown considerable light on the stelar evolution in both these families. As a result 
_ of research along this line, the occurrence of a comparatively simple stem-stele in M. 
_ duplex does not come as a surprise. In only one case has a simpler stele been recorded, 
viz. in Diplolabis rémeri. The elements of the stem xylem in M. duplex also show 
an archaic type of thickening on their walls—a reticulate type—and in this they are 
similar to the tracheides in the stem of Diplolabis rémeri, while quite distinct from 
the xylem elements in the xylem of other Zygopterid stems. In the inner zone of the 
xylem, however, some of the elements have scalariform thickenings, but this will be 
considered later. 
The mixed pith in this species does not play so conspicuous a part as in Ankyropteris 
corrugata and the other known Zygopterid stems. The radiating arms of parenchyma 
and tracheides so prominent in A. corrugata are certainly present in M. duplex, but 
they are not very well marked. In all cases they are closely connected with the 
emission of the petiole-trace, as, for example, the radial arm figured in Pl. II. fig. 15, a, 
which is shown to be intimately related to one of the protoxylem groups of the trace 
of Pl. I. fig. 7. Indeed, the insertion of the petiole-trace into the stem has probably 
been the cause of the stellate appearance of the latter in Htapteris di-wpsilon (Zygo- 
* P. BERTRAND, Etudes sur la fronde des Zygoptéridées, p. 206, Lille, 1909. 
