KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 57. N:0 I. 9 



than in specimens with straight leaves perpendicular to the stem. This latter fact inight 

 be taken to indicate that, in young stems and near the growing apex, the leaves may ha ve 

 been bent in towards the stem. On the other hand, there is a specimen represent- 

 ing the top of a stem fignred by Prof. Nathorst (1. c, pl. 4, fig. 7) which may belong 

 to this species and in which the leaves, though directed forward, are straight or even a 

 little bent from the axis. 



The specimens here described agree in all essentials with Dawson's type-specimens. 

 The latter display a similar variation in regard to the shape of the leaves and their 

 manner af attachment. Straight leaves going out at right angles are seen in the 

 Ganadian specimens, for instance in Dawson's pl. 13, fig. 146, though the enlarged 

 drawings, figs. 146 a and b, from the same specimen, represent falcate leaves. 

 Strongly falcate leaves are found, for instance, in Dawson's fig. 149, pl. 13; and in 

 some of his other specimens as well the leaves are sometimes a little falcate. Daw- 

 son's figures frequently show distinct leaf-scars, whereas no similar markings can with 

 certainty be observed in the Röragen specimens now described. It is not probable that 

 the scars in the Canadian specimens are caused by a natural shedding of the leaves: it is 

 difficult to understand why the leaves on the face of the impressions should regularly 

 have been shed, while those seen in profile at the outlines are still present. If the scars 

 are genuine and correctly drawn and not merely impressions of leaves perpendicular to 

 the bedding, which also seems possible, the corresponding leaves may have been broken 

 when the specimen was compressed. In regard to the disposition of the leaves, there is 

 also a close agreement between the Norwegian and the Canadian plants. The latter are 

 by no means always characterized by a verticillate arrangement of the leaves as is 

 also admitted by Dawson. In his second description of the species (1882 a, p. 104), 

 he says: »The main stems show the rounded scars approaching to a verticillate arrange- 

 ment . . . . » and »The smaller branches are more densely covered with leaves, which seem 

 to be spirally arranged ...... 



B. Specimens with unusually densely and regularly 

 placed leaves (pl. 1, fig. 8). — The impressions of two stems lying in parallel 

 position on the same slab which are shown in pl. 1, fig. 8, differ from those described 

 above, as well as from Dawson' s type-specimens, in the disposition of their leaves. 

 The stems are completely flattened, with a breadth on the impression of 15 mm. Only 

 very faint indications of rather short (6 — 8 mm. long) falcate leaves can be seen at the 

 outlines of the impressions ; but the surf ace of the latter is covered with markings which 

 are probably not scars but imprints of the thick basal portions of the leaves. These 

 markings appear as depressions on the slab figured but as elevations on the counterpart, 

 thus proving that it is only the peripheral tissue of one half of the cylinder which has 

 been preserved in the shape of two impressions, one corresponding to the outer and one 

 to the inner surface. The markings are more densely placed than the leaves of typical 

 specimens of Arthrostigma gracile; and it is only with some hesitation that these specimens 

 are described under the same name. The striking variation in the arrangement of the 

 leaves in this plant seems to indicate, however, that a difference in this respect need not 

 import a specific distinction. Very distinct parastiches appear in both impressions. 



K. Sv. Vet. Akad. Handl. Band 57. N:o 1. 2 



