KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 57. NIO |. 11 



The specimens shown in pl. 1, figs. 10, 14 — 17, may be mentioned in this connec- 

 tion, because they come nearer to those now described and to that figured by Kidston 

 than to any others. The reduction of the leaves just mentioned has gone still further 

 in this set of specimens, but the fact that there is a series of gradations in this respect 

 seems to indicate that all the different forms here brought to Arthro-stigma gracile may 

 fall within the limits of one species. In regard to the shape of the leaves the present 

 specimens agree fairly well with the one in pl. 1, fig. 9, and with those figured by Kids- 

 ton. On the whole, however, the leaves or thorns are shorter in respect to their breadth 

 or thickness: the leaves are only represented by very low thorn-like projections which 

 widen towards their bases so as to pass gradually into the surface of the stem. The re- 

 duction of the leaves, however, seems especially to have affected their number. In spe- 

 cimens of this kind the leaves or scars or other markings indicating their position are 

 always very distant : in some of the specimens there is no trace seen of them at all, as is 

 shown by some of the figures mentioned. The leaves are best seen in profile, at the con- 

 tour of the compressed stem. On the surface of the impressions they appear as pits or 

 hollows of a more or less rounded shape and indistinct outlines. 



The specimen in pl. 1, fig. 10, of which the counterpart is also preserved, shows 

 some irregularities in the surface markings which are difficult to explain. The rounded 

 impressions on the surface of the stem are here very distinctly circumscribed and have the 

 appearance of scars left at the shedding of the thorn-like leaves. The latter, however? 

 pass över so gradually into the stem that it seems improbable that they were actually 

 shed, and they are moreover still in place as seen in profile at the outlines of the impres- 

 sion. A curious thing, however, is that the markings are different, appearing sometimes 

 as depressions in the matrix, sometimes as elevations. In the upper part of the specimen, 

 at l t and 1 5 , there are concave markings, whereas those at 1, and l t in the lower part are 

 convex. In the counterpart, in which the same markings can be identified, the condition 

 is the reverse. The enlarged photographs, figs. 11 — 13, pl. 1, are all from this counter- 

 part, figs. 11 and 12 representing the counter-impressions of respectively 1, and l 2 infig. 

 10, fig. 13 of 1 3 . The specimen, at the time of fossilization, was evidently a very thin 

 cylinder of peripheral tissues covering alike the surface of the stem and the thorns. It 

 is possible that the difference between the markings may be explained in the following 

 manner. The cylinder may have been so thin that, as each thorn was imprinted as a 

 depression in the matrix, the thin wall of the opposite side of the cylinder was pressed 

 into the hollow base of the thorn and thus caused an elevation of the matrix of the 

 counterpart. 



The specimens of the kind now described show several instances of branching, as 

 for instance those figured in pl. 1, fig. 14, and in pl. 4, fig. 5. The branches are mostly 

 of nearly as great thickness as the main axis. They arise at a fairly wide angle but bend 

 rapidly upwards and assume a direction more or less parallel to the main axis. Spe- 

 cimens like the one in pl. 4, fig. 5, in their general appearance are strikingly suggestive 

 of rhizomes. It seems at any råte improbable that specimens of this kind could have 

 had an upright position in the air, especially as there is some evidence that the structure 

 of the stems was rather soft. 



