572 Benson.—Mazocarpon or the Structural Sigillariostrobus . 
sections tangential to the sporange (H. Cn. 518, 4-8) the lamella is very 
nearly level with the upper surface of the sporange. 
The lamella is shown well in Figs. 6, 5, and 18, and is indicated in 
others. 
Along the dorsal median line of the sporange a ridge is often seen 
which is doubtless due to the unusual degree of vegetative development 
the sporange has attained, which has made it capable of accommodating 
itself to the available space. 
In no Upper Carboniferous specimen has the number of spores per 
sporange exceeded eight. 
They will now be described. 
Section III. The ‘ Megaspore \ 
Owing to their size these can only be examined in section in petrifac¬ 
tions. The longest dimension of the spore shown in Fig. 3 is 1*94 mm. 
One in Fig. 1 is 1*7 mm. across ; another in Fig, 1 is 1*52 mm. across. 
The circular section in Fig. 9 has a 1*25 mm. diameter. 
The form is that of a tetrahedral or radially-symmetrical spore which 
on germinating has taken on lateral development in the restricted space 
available between the sporange-wall and the cylindrical subarchesporial 
pad. If we might use a homely illustration we might compare their form 
to that attained by a limited number of equal pieces of dough disposed over 
the upper surface and ends of a cylinder, after they had been submitted to 
such external pressure as would cause them to cover the cylinder without 
overlapping. 
If both the portions of the dough and the distribution of pressure were 
uniform the form of the pieces would show certain features in common. 
There would be a tendency to a concavo-convex form, and this would be 
most marked in those portions which had to * round ’ the ends of the cylinder. 
The problem in Mazocarpon is not quite so simple, however, for the 
sporange narrows rapidly towards the cone«axis and there are occasional 
instances of the invasion of the pad by the advancing development of the 
‘ spore ’. 
A remarkable feature in the orientation of the ‘spores’ is that the 
organic apex (triradiate scar) abuts directly upon the pad, i.e. is centro- 
scopic with reference to the whole sporange. One can only correlate this 
fact with the observed character of the pad tissue. This tissue was 
the chief source of water and food, and one must conjecture that it was 
a nutritive stimulus which brought about the definite orientation. The 
megaspores geminated while still, young and, growing in the residual sterile 
sporogenous tissue, became filled with a prothallial tissue which is often 
