KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 57. N:0 |. 19 



magnification in figs. 2 and 3. Tlie specimen in pl. 2, fig. 4, is interesting because it shows 

 both the rounded markings of the spines on the surface and impressions of the spines 

 themselves in profile to the left. Some of these specimens show a verv curious form of 

 sculpture of the outer surface. This is shown both in fig. 3, which represents an impression 

 of the outer surface, and in fig. 5 in which the outer part of the stem itself is seen in a 

 mineralized state. In fig. 3, which is thus the negative, this sculpture is found to consist 

 of a network of very deep, roundedly rectangular or more elongated meshes which mea- 

 sure 0,o5 mm. in breadth on an average, and are separated by elevated ridges. In fig. 5, 

 which shows the actual surface, the meshes appear as bulging elevations separated by 

 deep furrows. Round the bases of the spines the meshes become more elongated, with 

 a distinctly radial arrangement in respect to the spine, so that this peculiar sculpture evi- 

 dently continues out into the bases of the spines. It would be natural to regard the meshes 

 as representing the epidermal cells, but it is evident that the tissue presenting this sculp- 

 ture must ha ve been very firm. — In some specimens the scars are different from those 

 now described, being elongated instead of rounded. Such is the case for instance in the 

 stems shown in pl. 1, figs. 21 and 22. In these specimens the peculiar surface-sculpture 

 just described is absent; and this fact, together with the shape of the scars, renders it 

 probable that in these specimens a deeper level of the stem is exposed. That the diffe- 

 rence in the shape of the scars does not signify a specific distinction, but merely a diffe- 

 rence of preservation of the same kind of plant, is proved among other things by the occa- 

 sional occurrence of both kinds of scars in one and the same specimen. 



The disposition of the spines, as shown by the scars, appears to be rather irregular. 

 In some small areas fairly distinct parastiches occur, but in the near vicinity the arrange- 

 ment again may be irregular. In pl. 1, fig. 21, some steep parastiches are seen about 

 1,5 — 2 cm. below the lower bifurcation. In pl. 2, figs. 1 and 2, again the parastiches 

 form a wider angle to the axis, and as they are rather distant, an arrangement somewhat 

 similar to that mentioned above as occurring in Arthrostigma gracile is the result. 



Structure of the stem. 



In several of his papers on Psilophyton, Dawson describes specimens with pre- 

 served anatomical structure, of which some are stated to belong to this species. In 1859 

 (p. 478, fig. 1) he refers to Psilophyton princeps some stem-fragments which consist of 

 a central »axis of scalariform vessels », surrounded by a mostly destroyed zone of paren- 

 chymatous cells and a peripheral sheath of »elongated woody cells». As remarked above 

 (p. 15), however, no proof is given that these petrified specimens really belong to this 

 species. In the paper of 1863 (p. 465, pl. 18, fig. 22) Dawson describes another somewhat 

 similar specimen: the chief difference is that the central strand is stated to be hollow, 

 whether this be due to the occurrence of a pith or to detorioration of the vascular tissue. 

 This specimen shows some minute elongated markings and scattered circular areoles 

 similar to those in the impression specimens of the real Psilophyton princeps (ef. above, 

 p. 15). The identity, however, is open to doubt. The same is the case, in a still higher 

 degree, with the petrified remains of Psilophyton described and figured in the report of 

 1871 (p. 67; pl. 11, figs. 133, 134; pl. 20, figs. 240—242). These remains are not all 



