108 Proceedings of the Royal Physical Society. 
not think that these spine-like structures are the bases of 
leaves, as has been suggested, but are the leaves themselves, 
though developed in a very rudimentary form, as in Psilo- 
phyton. Though the leaves now appear as flattened thorn- 
like outgrowths, that they were originally circular is proved by 
the circular scar they leave when removed from the stems. 
When the stems are denuded of their spine-like leaves, the 
scar they leave gives the fossil very much the appearance of 
a Stagmaria. 
The surface of the stems are irregularly striated longi- 
tudinally, and bear tubercle-like prominences—the points 
to which the leaves have been attached. These prominences 
in their present condition are, probably, more elevated than 
they were in life, by the shrinkage of the bark. Owing to 
compression of the fossils, the disposition of the leaves is 
not very clearly shown on most of the specimens, but on 
the best preserved they are seen to be arranged in 
spirals. 
The characters possessed by these fossils clearly show 
their position to be in Dawson’s genus Arthrostigma. 
It is true that Dawson mentions that the leaf-scars are 
arranged in whorls, and on the great majority of the 
specimens it is impossible to determine satisfactorily their 
arrangement, and even on Dawson’s figures of Arthrostigma 
gracile this point is obscurely illustrated,—on some of his 
examples the leaves do appear to be arranged in whorls, 
but on others they seem to be spirally disposed; so this 
point, as to whether the leaves were in whorls or placed in 
spirals, cannot at present be insisted on as offering any 
distinguishing character. Then again, the majority of the 
spine-like leaves on Dawson’s figures appear to be straight, 
while a few are falcate; on the Perthshire specimens, the 
majority are falcate, while a few are straight. 
There is, therefore, no definite point of difference between 
the Canadian and Scottish examples in any of these characters. 
Dawson further says, that “Arthrostigma gracile would seem 
to have been a small shrubby plant, with stems not exceed- 
ing an inch in diameter.” 
Some of the Perthshire stems are certainly a little broader 
