50 
T. G. HALLE, 
(Schwed. Südpolar-Exp. 
judge from Heer’s specimens, and does not, on the whole, show any great re- 
semblance in habit. There is only one specimen of the species, figured by Thomas 
(1911 a, pi. 7, fig. i) from Southern Russia, that shows the same shape and size of 
the frond as the Antarctic form. This specimen comes fairly near N. tenuinervis 
Nathorst, which is distinguished from N. orientalis also by the narrower shape of 
the frond. Both species, also the just mentioned specimen of ThomaS, differ from 
our form by a much denser venation (30 — 40 veins in one centimetre against 15 — 20 
in the present spçcies) and generally straighter veins, given off at a more open angle. 
In regard to the venation the closest agreement is with Nilssonia densinervis 
CFont. ) from the Potomac group of N. America (Berry, 1911, p. 362; pis. 57, 58). 
In this species there are, according to Berry, 10 — 30 secondary veins in a length 
of a centimetre; and his figures show the veins to be about as dense as in the 
Antarctic fronds. They are, however, straighter and at a more open angle to the 
midrib. In Nilssonia densinervis, moreover, the fronds are only rarely entire; and 
since they appear to be constantly so in the Antarctic form, it seems necessary to 
keep the latter distinct from the other species, though this seems to be the one most 
closely related. - 
Genus Pseudoctenis Seward. 
This genus was instituted by Seward (1911 a, p. 691) for fronds intermediate in 
some respects between Ctenis and Pterophylluni. The habit of Pseudoctenis is that 
of the former genus, to which the resemblance is on the whole closer. The difference 
is mainly in the venation, which is not regularly anastomosing as in Ctenis but 
occasionally may show some few cross-connections. In regard to the absence or 
rareness of anastomoses, Pseudoctenis comes nearer to Pterophylluni, from which 
genus it differs, however, not only in the occasional occurrence of cross-veins, but 
also in a generally coarser venation. 
It is evident that the genus is not very distinctly marked off from any of the 
allied genera, and that it will often be difficult to decide whether a certain form 
should be referred to it or not. Yet it seems to serve for practical purposes, since 
it affords a convenient place for forms intermediate between Ctenis and Pterophylluni. 
Berry has recently (1911, p. 347) instituted a new genus, Ctenopsis,iox ■à.ixowà 
of the Potomac flora, previously known as Ctenophylluni latifoliuni Fontaine (1889, 
p. 175). The form is stated greatly to resemble Ctenis, but to differ from that genus 
in the apparent absence of anastomoses. Seward’s paper on the Jurassic flora of 
Sutherland (1911 ci), in which the genus Pseudoctenis is described, had not appeared 
at the time when Berry instituted his new genus, but on becoming acquainted with 
that work Berry adds a note to his description, stating that both genera are closely 
