McLean . — On the Fossil Genus Sporocarpon . 77 
vS. comp actum, and questionable juveniles of W tubulatum and S. pachyderma 
have been found, but there is nothing to add on this point to what was 
said at the beginning of my first account, i. e. that the spores are apparently 
asexual autospores, developing directly into mature individuals by increase 
of size. Minute vesicles are apparently responsible for the markings on 
their surface, and these are thought to develop into the envelope ‘ cells ’ of 
the mature organism. 
Average dimensions : 
Total diameter, 320 ju. Diam. of cells: Base, 1 6*5 /x. 
Internal sphere, 220 \x. Near top, 22 ju. 
Length of cells, 50-60 ju. Diam. of sporoids, 25-45 /a. 
(ii) 5 . tabulation . Williamson, ‘Phil. Trans.’, pp. 169-349, 1878. 
This species is anomalous in being oval, though the difference in length 
of the two axes is not very considerable. All the same it affords a parallel 
to Zygosporites oblongus , although not so markedly elongated, and thus 
helps to illustrate the homogeneity of the group. Taking an average of 
the few specimens available, the greater diameter is about one-eighth more 
than the lesser. 
The structure of the envelope in 5 . tubidation is superficially similar 
to that in the preceding species, so that it is not surprising that it was 
originally named as a variety of 5 . compaction } In the young condition 
there are contiguous tubular vesicles all over the exterior surface, their 
long axes disposed radially, as in the first species, though here the vesicles 
are nearly five times as long as broad. They are slightly broader distally, 
and their outer ends, instead of being sharply pointed, are smoothly rounded 
off, without any papillae or openings of any kind. 
At the base of each such cell is a well-marked pore communicating 
with the interior of the sphere. Indeed in this species this form of opening 
is better shown than in any other. There can be no doubt that free 
interplay of living material existed between the interior cavity and the 
peripheral cells during life. 
It is remarkable that both in this and the former species the cells of 
the envelope are very constant in size and regular in their adjustment each 
to the other. Just as in S. compaction, the vesicles of the envelope matured 
into long spines ; but here we have definite evidence that they all elongated 
as in N. elegans , although probably in succession, not simultaneously. 
Maturation of the spines was accompanied by swelling out into 
a globose bulb, at about the summit level of the young vesicles, in such 
a way as to thrust aside the neighbouring ones and clear a space round 
the proximal portion of the new spine. The apical half tapered regularly 
to a sharp, solid point, without any sign of perforations on its' wall, the 
whole mature spine being more than twice as long as its parent vesicle 
1 Williamson, in his slide catalogue, not published. 
