8o 
McLean. — On the Fossil Genus Sporocarpon. 
young stage must have been very like the young stage of the former species, 
and may have been confounded with it. 
The interior capsule and sporoids have been found very well preserved, 
with abundant periplasm. In the spines of this species, as well as others, 
I have sometimes seen very small 
can hardly be true spores in such 
commensal protophytes, analogous 
found in modern Radiolaria. 
Dimensions : 
Total diameter, 480 /x. 
Diam. of sphere, 360 \x. 
Length of spines (base to 
tip), 180-240 \x. 
spheres, not unlike the spores. They 
a position, and I believe they may be 
to the testiculate flagellata sometimes 
Width at swelling, 25-35 ix. 
Width at base, 25-35 /x. 
Width of constricted part, 8-9^. 
Height of colonnade, 50 -65 /x. 
§§§ Diadematis. 
S. cellulosum . Williamson, ‘Phil. Trans.’, pp. 169-347, 1878. 
In this subgenus the true structure of the envelope is in some doubt. 
So far as can be judged there appear to have been divaricate spines in 
groups round the periphery, each group having something of a tiara-like 
appearance in side view. I have never seen any specimen in which these 
spines were fully developed, and it is open to question whether any 
elongation took place further than what has been observed. In this 
condition they form stout, comparatively short tubes, with dark and rather 
thick walls, blunt and open distally. Each group consists of longer tubes 
in the middle, the remainder declining in length outwards, in both directions, 
giving the impression of a kind of sorus with basipetal development. 
That development did actually run in this order seems evidenced by 
the way in which the neighbouring elements of the envelope are deflected 
from the truly radial position, pushed away as it were by the elongation 
of the elements of the sorus. Between these widely separated groups of 
spinous processes the envelope is composed of similar, but quite short tubes, 
evenly distributed. In tangential section all appear to be hexagonal and 
close fitting, but in radial view the appearance is that of alternating cells 
and vacant spaces. 
Running tangentially between all these tubular processes are several 
series of fine dark lines. These may be either very fine tubules themselves, 
or thickening bands upon the radial cell-walls, or, more probably, pairs of 
them may be the edges of comparatively stout tubes in longitudinal section, 
or they may be the walls of true cell elements, filling in the interspaces 
between the radial tubes. As I said before, the specimens available have 
not been sufficient to elucidate the point, though I incline to the second 
view. If they are really cells, there would be afforded a welcome link with 
the structure of the following species, which otherwise is extremely isolated. 
