KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 51. N:o 3. 21 
to form a fine network of short and wide, often rounded meshes. The areoles be- 
tween the secondary veins and the midrib are also filled up by this network, to the 
formation of which contribute not only the tertiary and quaternary branches but 
also other veins of the same thickness arising direct from the midrib. 
The present species shows, as do others of the same genus, a certain habitual 
resemblance to Laccopteris. "There is no evidence, however, that the Patagonian spe- 
cimens represent segments of pedate fronds, as do the corresponding specimens of 
Laccopteris. It is possible that the frond was bipinnate, but there is no evidence of 
this, and it may be regarded for the present as only pinnate. The venation, which 
was not clearly shown in the previously known species of Nathorstia, agrees fairly well 
with that of some species of Laccopteris, both in regard to the open angle formed by 
the secondary veins with the midrib and the anastomosing of the ultimate branches. 
In addition to this resemblance to Laccopteris shown in the vegetative charac- 
ters there is also a similar arrangement of the sori in one contiguous row on each 
side of the midrib. A close examination, however, reveals a very different structure 
of the sori, which renders it necessary to separate the Patagonian fossil from Lac- 
copteris and to refer it to Nathorstia HR., as this genus is characterized by Prof. 
NATHORST. In his description of Nathorstia latifolia NATH., from the Cenomanian of 
Greenland, Prof. NATHORST has shown that the sporangia of each sorus are fused to 
form a real synangium containing 18—20 wedge-shaped loculi. A very similar struc- 
ture is found in the Patagonian species. 
The sori are circular on the impression, with a diameter of 1—1,5 mm. They 
are often densely crowded, however, so that they come to border on each other with 
straight lines. They are placed between the secondary veins but their exact position 
in regard to the venation cannot be ascertained: it is not possible to decide whether 
they are placed on the first tertiary vein arising from each secondary vein or on 
veins originating direct from the midrib. Each sorus forms a synangium which, of 
course, is completely flattened in the specimens but may have been fairly high in 
the natural state. The synangium has a central receptacle. From this radiate to 
the periferal wall of the synangium 12—15 fine straight lines which divide the synan- 
gium into an equal number of wedge-shaped loculi. This structure, as shown in 
pl. 1, figs. 4—7, proves conclusively that the sorus consists of a synangium and not 
of free sporangia. The radial line dividing each two adjacent wedge-shaped areas 
cannot be anything but the common wall of two adjacent loculi, and it is clearly 
seen in the figures that it joins the tangential wall surrounding the whole synangium. 
On an examination of the two counterparts it is seen that the sori have about the 
same aspect on both of them. This appears to indicate that the synangia have been 
split in such a manner that their bases adhere to the piece which shows the impres- 
sion of the upper side of the frond, their tops or roofs to the counterpart presenting 
the impression of the lower side. Both impressions thus show the internal structure 
of the synangia and some of the spores still present in the loculi. As the roof of 
the synangium is not shown, it is impossible to decide in what manner its opening 
was effected. On the whole, the structure is identical with that of Nathorstia lati- 
