McLean . — On the Fossil Genus Sporocarpon. 85 
which every vesicle was developed into a spine (one of Williamson’s 
specimens approaches this condition), we would possess a feasible working 
hypothesis of a prototype from which not only the remaining species of 
Sporocarpon may be derived, but which might give indications (chiefly 
through vS\ cellulosum ) of the origin of the more specialized genus 
Traquairia. 
Relationships to Other Protozoa. 
The comparison of the genus as a whole with living Protozoa is not 
easy, for there is no resemblance in an aggregate of particulars such 
as to point indubitably to one group with which detailed morphological 
homologies might be shown. There are indications of affinity to both 
Foraminifera 1 and to Radiolaria, but especially the latter. Not that these 
present real homologies with the Traquairidae. At most it can be said 
there are suggestive analogies. Chiefly interesting, perhaps, is the central 
capsule, in the Radiolaria the seat of the nucleus and of sporulation. 
A fundamental divergence is the absence in these fossils of any 
system of perforations of the capsule wall, unless the fine punctations 
occasionally observed are to be so interpreted, in which case their affinities 
would obviously be with the primitive group of Peripylaria, possessing an 
evenly porose capsule. Association with the Peripylaria raises naturally 
the question as to whether we are not dealing in a Sporocarpon sphere with 
a colony of individuals, after the fashion of Collozoum inerme , each sporoid 
being then regarded as the homologue of a complete central capsule. 
I have already indicated that the balance of evidence is against this opinion, 
but it cannot altogether be discounted. The spores, again, present 
difficulties, for the spores of Sporocarpon are furnished with a palpable wall, 
enclosing protoplasmic relics, while those of modern Radiolaria are quite 
unsubstantial and indeed self-motile. Here a comparison with Foraminifera 
may help us out, for among them the reproductive amoebulae surround 
themselves with a minute one-chambered cell as the starting-point of a new 
individual. How the spores were exserted from the interior chamber of 
the organism there is no evidence to show, unless they may be imagined 
to escape by the dissolution of the envelope . 2 This envelope appears to be 
held together chiefly by its attachment to the sphere wall, but the sphere 
wall may not have been so substantial as it appears, for it may be looked 
upon as the correlative of the dark fatty assimilative layer of protoplasm 
closely surrounding the capsule in the Radiolaria, and thus perhaps much 
more readily disintegrated than a solid non-plastic membrane would be. 
1 The relationship to Foraminifera has not been considered here so fully as that to Radiolaria, 
but it may be pointed out that the genus Polytrema has several features suggesting affinity with the 
present group. 
2 But vide supra , p. 76, and Fig. 9. 
