418 MISS T. L. PRANKERD ON THE STRUCTURE OF 
The grains are oval in form, and their dimensions have been given by 
previous writers, Му measurements give an average of 72x53 y ; but this 
is certainly an underestimate of the length, as the grains are not all eut in 
the planes of the major axes. No doubt the grains of those days varied as 
much among themselves as do those of ours, and some reached as much as 
90 ш in length. 
With regard to structure, little more can be made out than that the grains 
had thin places in their walls. Some appearance of а network 1s frequently 
seen (Pl. 99. fig. 5 band PI. 28. fig. 19), but in no single case does it 
seem possible to refer this with certainty to а cell reticulum; it seems quite 
as likely to represent surface markings. Still less is there any trace of 
germination. In В. 38 (РІ. 22. fig. 5 а) there is in the pollen-chamber an 
oval body measuring only 68x35 и, which, instead of showing the usual 
reticulate markings, is of a uniform dark brown with extremely fine dots. 
It is perhaps a foreign pollen-grain. 
A word may be added as to the position of the grains. Not any have ever 
heen found in the sinus between the lagenostome and the canopy. Within 
the pollen-chamber they tend to occur towards the base, and in five cases 
have been found resting on the megaspore membrane or where this would have 
been in life. Sometimes they appear more or less embedded in the tissue 
of the central cone, a state of things probably due to post-mortem changes 
in the position of the latter. К. 28 affords the solitary example of a pollen- 
grain occurring in the neck of the lagenostome, and in В. 30 a (РІ. 24. 
fig. 84) we have a grain outside the seed situated in an apical groove. Ina 
fow other cases bodies have been seen near the apex which may have been 
pollen-grains, but in the surrounding débris it is very difficult to be certain 
on this point. 
E. The Prothallus. 
Williamson referred to а central body in one of his sections as possibly 
a prothallus, but more probably an effect of mineralization, and Chodat 
asserts that the existence of a prothallus has not been demonstrated in 
Lagenostoma (5, p. 30). The preparation from which Williamson’s figure 
((20) pl. 10. fig. 68) was drawn has not been traced ; but whether it showed 
a prothallus or not, we now have, in addition to the numerous sections which 
show the thick megaspore membrane, no less than four slides (R. 24, R. 23, 
R. 42, and R. 44) which have strong claims to have preserved the actual 
substance of the prothallus. 
The megaspore membrane, usually more or less contracted and crumpled, 
has very much the same appearance as that of the nucellus, with which it 
is often in contact, sometimes so closely as to be indistinguishable. Again, 
like the nucellar membrane, it has frequently a pitted appearance. 
