Bd. III: 14) 
THE MESOZOIC FLORA. 
49 
undulating; but even that may be due to the preservation only. The lamina is often 
keeled, with the upper surfaces of the halves forming an open angle with each other. 
Apart from this, too, there can often be seen a distinct difference between the upper 
and the lower side. On impressions of the latter, the midrib, which is very stout 
and sometimes may measure as much as 5 mm. across, appears as a shallow groove, 
at the sides of which the lamina appears sharply cut off. This is due to the fact 
that the lamina is attached to the upper side of the rachis, as can be seen from an 
examination of the impressions of the upper side. In these, the midrib projects 
more or less as a low ridge, and in favourable cases the veins may be seen to con- 
tinue in over the edges of the latter and join the upper side of the rachis. This 
feature is indicated in the text-fig. ii b: but it can by no means be distinguished in 
all impressions of the upper side. 
It is well known in other species of Nilssonia that the prominent vein-like ribs 
running from the rachis to the margin, do not represent the actual veins but eleva- 
tions of the tissue of the frond projecting between them. In some of our specimens, 
the fine veins themselves can be seen; in other cases, however, their course is marked 
sufficiently by the ribs. The veins start at a fairly wide angle and, a short distance 
from the midrib, arch strongly forwards, the curvature increasing towards the mar- 
gin. They are fairly dense, but in this respect also there is some variation; the 
average seems to be nearly two to the millimetre. The veins are all equal and are 
normally simple. It is only in very exceptional cases that forking occurs, in one 
single vein perhaps in a number of specimens. Sometimes, though still more rarely, 
two veins join tow'ards the margin. 
The species has somewhat the habit of a Tœniopteris, differing from most Nils- 
sonias in the entireness of the lamina which shows no trace of segmentation. That 
it belongs to the latter genus is proved, however, by the characteristic attachment 
of the lamina to the upper side of the rachis and also by the fact that the veins are 
normally simple. That in single cases forking may occur does not weaken the latter 
argument, since it was shown by Nathokst already in 1876 (p. 42) that the veins 
in Nihsonia polyinorplia may occasionally divide. The arching of the veins and the 
relation of the lateral ribs to the latter repre.sent also characteristic features of Nils- 
sonia. 
This Antarctic form cannot be referred to any one of the existing species of 
Nihsonia, differing from some in the shape of the lamina, from others in its vena- 
tion. In regard to the habit, the closest agreement should be found in species with 
entire lamina, since that feature seems to be constant in the Antarctic fronds. The 
species to be brought into comparison are chiefly N. oricnialis Heer (1878, p. 18; 
pi. 4, figs. 5 — 9), and N. temiinervis Nathorst (1880, p. 35; cfr. Seward 1900, 
p. 230). The former species is smaller and has much shorter fronds, at least to 
7 — 122943. Sc/nvedisehe S'ltdpolar-Expedilion igoi — içoj. 
