KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 57. N:0 |. 33 



either older or younger. The only evidence afforded by the geology is the fact that the 

 deposit is younger than the folding of the Caledonian mountain range, i. e., not older than 

 the beginning of the Devonian period. There are no traces of any animal fossils, and the 

 only palaeontological information on the geological age is therefore that offered by the 

 plant-remains. 



Arihrostigma gracile is a well characterized species which may with confidence be 

 identified in different deposits. It is best known from the Devonian of Eastern North 

 America were it occurs at Gaspé and at Campbellton. The sandstones at Gaspé, which 

 have been shown by Sir W. G. Logan to rest conf ormably on the Upper Silurian limestones, 

 probably represent the whole of the Devonian period (Dawson 1871, p. 7). Arihro- 

 stigma gracile, according to Dawson, is confined to the lower part of this series. In the list 

 of species in Dawson's Report of 1871 (p. 85) the species is recorded from the Lower 

 and Middle Devonian of Gaspé, but in the description (p. 41) the horizon is only given 

 as Lower Devonian. In another place (p. 42) Dawson says: »it seems in Gaspé to be 

 limited in its upward range to the lower part of the Middle Devonian, as if it were then 

 a form verging on extinction». The locality at Campbellton is stated to belong to the 

 Lower Devonian (Dawson 1882 a, p. 102, 104). The horizon on which Arihrostigma gra- 

 cile is found in Scotland is, according to Kidston, that of the Lower Old Red Sandstone. 

 Finally the Belgian specimens which are referred by Crépin (1875) to Lepidodendron 

 Gaspianum Daws., but of which some appear to belong to Arihrostigma gracile (see 

 above, p. 13), occur in deposits which, according to Crépin (1. c, p. 7), are considered 

 by all Belgian geologists as Lower Devonian. Some specimens of Psilophyton spinosum 

 (Krejci) Potonié & Bernard and P. bohemicum (Stitr) Potonié & Bernard from 

 the Middle Devonian of Bohemia (Etage H of Barrande), as mentioned above, are some- 

 what similar to Arihrostigma gracile, but they are probably not identical. All sufficiently 

 established facts regarding the distribution of Arihrostigma gracile thus agree in assigning 

 a Lower Devonian age to this species. 



Psilophyton princeps has been recorded from different localities and from different 

 horizons. With the definition of the species adopted in this paper we can only accept as 

 reliable evidence of its distribution such statements as are verified by illustrations or 

 descriptions of sjmie-bearing stems. In the only paper by Dawson (1871) in which any 

 satisfactory evidence of the occurrence of this species is given, he records it from the 

 Lower and Middle Devonian of Gaspé and from the Lower Devonian of Campbellton, 

 but the material from the latter locality is not accepted by White as evidence of the 

 occurrence of this species. Dawson speaks of it as especially characteristic of the Lower 

 Devonian. It is further stated to occur already in the Upper Silurian (1871, p. 66), but 

 the specimen figured from this horizon affords no sufficient proof of identity. In the 

 list of species in the Report of 1871, it is also recorded from the Upper Devonian, but 

 this statement is not borne out by any illustrations. The same remark applies to other 

 localities in Canada and in the United States from which the species has been reported, 

 with the exception of the localities recognized by D. White, and mentioned above, in 

 the descriptive part. As remarked above, it is not possible to ascertain from published 

 illustrations whether any of the numerous statements regarding the occurrence of Psilo- 



K. Sv. Vet. Akad. Handl. Band 57. N:o 1. 5 



