72 On the Structure of Exogenous Stems. [ToS. aSTS^ 
two elements of external discs and internal deposits whicli, when 
combined, constitute the coniferous vessel. The Darwinian would 
find no difficulty in concluding that the pollen of one of these 
plants might fertilize the ovules of the other, and thus produce, at 
some later period, structures in which there combined the two 
elements that are severed in the vessels of Dadoxylon and Dicty- 
oxylon. 
It is evident that both the above genera possessed in some 
instances, if not in all, piths of the Sternbergian type. Fig. 11 
represents a beautiful vertical section of a Dictyoxylon from Mr. 
Butterworth's cabinet — in which the Sternbergian pith is very 
well displayed — the discoid laminae extending completely across the 
medullary cavity. This specimen is also remarkable for the regu- 
larity of the laminae, as well as for the peculiar thickened aspect 
their section exhibits near the walls of the medullary space. 
When we consider how abundant these large exogens — espe- 
cially the Dictyoxylons — are in the Coal-measures, it becomes 
remarkable that we have hitherto been unable to identify either 
their foliage or their fructification. I cannot believe that the latter 
assumed the form of recent cones, or such would have been common 
amongst other Carboniferous fossils, because of the readiness with 
which such structures are preserved. This is shown by the 
abundance of Lepidostrobi in these beds. It seems to me not 
improbable that Trigonocarpon may have constituted the fruit of 
one of these genera. Dr. Hooker has already expressed his convic- 
tion that it was the fruit of a conifer allied to the modern Salis- 
buria : from its abundance in many districts, it must have belonged 
to one of the more common trees of the Carboniferous period. This 
would point to Dictyoxylon, which is abundant in the Lancashire 
districts, where Trigonocarpons are common, as constituting the 
parent stem from which the latter had fallen. 
I would observe, in conclusion, that I have very little confidence 
in any determinations respecting fossil plants, excepting such as are 
based upon internal structure: all the modern classifications of 
living plants rest upon this basis, and it is the only safe guide 
amongst fossil ones. Of course its indications require to be 
checked by considerations of external form ; but the former must be 
primary, and the latter but secondary aids. We rely upon the 
former when the latter is not to be obtained, but only then. 
