McLean. — On the Fossil Genus Sporocarpon. 79 
Dimensions : 
Total diameter, 55 j a. 
Diameter of sphere, 28 ji. 
Length of cells, 14 /x. 
Breadth of cells, 10 //. 
§§ Fredaia. 
S. elegans. Williamson, ‘ Phil. Trans.’, pp. 169-348, 1878. 
This was taken in my last paper as the type species of the genus, but 
I now refer S. compaction . to that position, as the least specialized type. 
The present organism has an envelope formed of tapered spines, with 
a median and a proximal swelling, between which — that is, in the lower half — 
the shaft is constricted to a slender stalk. The median and basal swellings 
are contiguous all over the sphere, so that there is formed a ‘ colonnade ’ 
of the slender portions of the shafts and the continuous space which lies 
between them. As in 5 . tubidatum the spines must have been somewhat 
brittle, for only a few survive at full length even in the best-preserved 
specimens. They were broken off all about the same level, just above the 
median swelling, and it is easy to see, by the broken stumps which form 
the greater part of the envelope in most specimens, that originally each cell 
of the envelope formed a spine. Where the ‘ cells ’ of the colonnade touch 
one another their walls are thickened, so that where the spines break off, 
just above this level, there is a thickened honeycomb-like framework left 
behind at the outer surface of the colonnade layer. (Sec PI.* VIII, Figs. 1 
and 3.) Occasionally spines may be observed flexed, but not often. The 
walls of the spines are perfectly smooth and free from perforations, but the 
tips are often bifid or trifid, with widely divergent segments, a character 
which has its counterpart in Traquairia. The lumen connects with the 
interior of the sphere by a single pore. 
Williamson believed the spaces of the ‘ colonnade’ to be filled by thin- 
walled cells, relying upon the appearances shown in tangential sections. 
I myself, finding that the walls of these ‘ cells ’, cut in tangential sections of 
the envelope, were double throughout, came to the conclusion that they 
were really the remains of a system of tangential tubuli, homologous with 
those of Traquairia , connecting up the proximal ends of the ‘ colonnade ’ 
units. The spaces of the ‘colonnade’ are not, I believe, occupied at all, 
but the appearance of traversing walls or tubuli may possibly be due only 
to the overlapping of the margins of the basal swellings, which can easily 
be observed in transverse section, and which may lead to this appearance of 
tangential connexions when seen from above. 
The species is a fairly common one, and is often singularly plentiful 
when met with. Good specimens well deserve the specific name. Curiously 
enough, no specimens have been found which throw any light on the 
development of 5 . elegans , although we have good evidence of the history 
in both 5 . compactum and 5 . tubulatum. There is little doubt that the 
