IN THE LOWER OLD BED SANDSTONE OF SCOTLAND. 219 



from which the leaves have disappeared. The leaf is not clean-cut 

 on the upper margin, but has something like a cicatrix scar. 



" As to their name, it is probably the larger stem of a plant like 

 Dawson's Psilophyton princeps, with which it agrees in structure 

 &c. No doubt his Arthrostigma and Cyclostigma are the same things ; 

 but we yet want light as to the true nature of these Devonian Ly- 

 copods." 



The preceding facts comprise all the in formation we at present 

 possess bearing on these interesting fragments. Beyond the probable 

 dichotomous method of branching, we are unacquainted with any of 

 the broader characters ; neither do we know any thing of the fructi- 

 fication. The plentiful manner in which the matrix is traversed, gives 

 rise to the hope that the discovery of more perfect specimens is only 

 a matter of time and careful search. Under these circumstances we 

 content ourselves with appending a short provisional description 

 under the name of Psilophyton (?), sp., in accordance with the sug- 

 gestion of Mr. Carruthers, as given above. 



Psilophyton (?), sp. 



Stems branching dichotomously, and covered with spirally oblique 

 lines of short, rigid, pointed and slightly curved thorn- or spine- 

 like projections, the persistent bases of the leaves, or perhaps the 

 leaves themselves. Axis composed of a slender column of vascular 

 tissue, surrounded by a cylinder of cellular tissue. 



4. Geological Notes on Horizon. — It now only remains for us to 

 indicate the horizon of the plant-bearing flagstones within the limited 

 district where these have been met with. This region is but the 

 smaller end of the largest Scottish area of the Lower Old Red Sand- 

 stone formation ; and the structure of the greater part of that area has 

 yet to be mapped out. It is not improbable that similar flagstones 

 containing similar fossils may yet be discovered in other places and at 

 various horizons. It would be premature in the mean time to correlate 

 the divisions of the strata, within the limited area in question, with 

 those of separate and distant contemporaneous areas, considering the 

 paucity of fossil and other evidence. 



If we traverse a line parallel to the course of that part of the river 

 Teith lying between Callander and Stirling, and about four miles to 

 the north-east of that river, we meet, in a great synclinal trough, 

 with an immense thickness, though by no means the whole, of the 

 Lower Old Red Sandstone strata. The highest or axial beds of this 

 trough cap horizontally the tableland of Uamh Mhor and Uamh 

 Bheag, while the subjacent beds crop out on the south-east, with a 

 dip gradually increasing to about 30° at the Sheriffmuir, and on the 

 north-west with a dip which rises rapidly to very near the vertical 

 at the heads of the Keltie and Ruchil Waters, where a great fault 

 brings up the Silurian rocks of the Highlands. 



As will be readily understood, from the trappean and conglome- 

 ratic nature of a large portion of the formation, especially in its 

 lower parts, the strata vary very widely in character and in thick- 



