84 McLean . — On the Fossil Genus Sporocarpon . 
that this is the mother-cell of a colony, and that the ring of radial elements 
is the precursor in development (from the spore) of the tangentially arranged 
tube network, and therefore only found in the mother-cell, while absent from 
the proliferous outer daughter-spheres of the colony. Such a scheme 
affords a useful hypothesis, and serves to exhibit a possible connexion of 
this anomalous form with the typical envelope of radial vesicles in § En - 
sporocarpon. It will be desirable to advert to these possible affinities 
later on. 
A very probable spore, with irregular wall, has been found in close 
proximity to a large nest of individuals (see PL IX, Fig. 14). 
Dimensions : 
Total diameter, 330-390 fi. 
Diam. of sphere, 240-270 /x. 
Average thickness of integument, 60-90 /x. 
Average width of tubules, 8-16 /x. 
Interrelationships. 
It would be fallacious to try to unite these few species into anything 
like a phyletic sequence, but it would not be undesirable to emphasize here 
the points in which they seem to connect with one another. 
Taking S. compaction as the standard, we can see that separation ot 
the vesicles from one another and grouping of the scattered spines into 
4 sori ’ would produce a type resembling X. cellulosum. Again, complete 
development of the vesicles into spines, as shown in the closely allied 
X. tubulatum , leads us to the X. elegans type, where the constriction of the 
basal moiety of each spine — indicated in one peculiar example of X. com- 
paction itself (T. 8, in my own possession) and normal in X. tubulatum — has 
been carried to the extreme. As before stated, X. asteroides stands some- 
what apart. Its nearest relative would appear to be Traquairia stellata, 
R. Scott, where the spinose arms are divided into cellular compartments,, 
but I think the two are really parallel developments, and not genetically 
connected. S’, asteroides might be reached by a proliferation of the 
juvenile vesicles of S. compactum into a pseudo-parenchyma, in place of 
their normal individual development into spines. 
S. pachy derma , in the light of the specimen with an inner radial 
investment of cells, is best regarded as a derivation from the S. compactum 
form, in which a colonial habit has been established and a general envelope 
formed by the prolongation and anastomosis of the distal portions of the 
radial envelope cells into a complex outer stereome of tubules, beyond 
which their free apices may have projected. Obviously this yields points 
of contact with the envelope of Traquairia , where ramification of the spines 
is general. But the envelope is formed by the anastomosis of proximal 
rather than distal ramifications. Postulating a type of X. compactum in. 
