McLean. — On the Fossil Genus Sporocarpon. 87 
quairidae may have sprung from some similar group of Peredinian affinities, 
with spores possessing a hard cell-wall, in place of the lower, flagellate type 
represented by the spores of Radiolaria. It has been maintained all along 
that their homoeomorphy with the latter group is rather the result of parallel 
descent than of homology. 
Sporocarpon pachyderma has particular affinities of its own with the 
Keratosa group among the sponges, and represents an advance in their 
direction, which brings it into relation to a parallel living group of 
Protozoa, the little-known Xenophyophoridae (6), which also, though widely 
asunder, have developments tending similarly to a sponge-like facies. 
I have previously suggested that the present group represents a line 
springing from common ancestors with the Radiolaria, probably not very 
far back from the present level of that class, the modifications being 
traceable to life in shallow, highly organic waters with a low saline content. 
Such habitats have not often been extensive in geological history, but as 
the Carboniferous Coal Measures are the only period of the sort sufficiently 
closely investigated, it is impossible to deny the existence of similar 
organisms at other epochs similarly constituted. Not improbably Mesozoic 
and Cainozoic coal deposits may yield allied or identical forms when they 
have been more fully examined. At present, however, so far as we can 
tell, they are exclusively Palaeozoic and* limited to the Coal Measures, 
though on those horizons they attained a wide distribution. The mineralized 
skeleton of the pelagic Radiolaria is by no means an insuperable difficulty 
in the way of associating the groups, since such skeletons are also found in 
several isolated groups, for example among the Diatomaceae and the 
Silicoflagellata, without the occurrence of anything but an organic test in 
their nearest relatives. Fossil Radiolaria are found extensively in Lower 
Palaeozoic rocks, even the lowest, notably in the Lower Culm of Devon 
and Somerset ( 7 ) and in the European Culm in general, almost at the base 
of the Carboniferous succession (8). Use has been made, by geologists, of 
their presence in these rocks to argue very deep water conditions at the 
opening of this geological period, and a gradual shoaling until the Carbo- 
niferous Limestone is reached, with subsequent elevation and the com- 
mencement of the Upper Carboniferous rocks. 
There can be little doubt of the affinity of the organisms figured by 
Hinde and Fox ( 7 ) both with Radiolaria and with Traquairidae, though 
clearly their skeletons have undergone resolution and replacement, so 
much so indeed that the generic and specific assignations awarded them 
are more than dubious. The interesting point at the moment is the 
presence of a class of organisms resembling modern Radiolaria, widely and 
plentifully distributed in the seas preceding the formation of the Coal 
Measures. From this organismal reservoir it is, as I conceive, that the 
Traquairidae were drawn, during the subsequent shallowing of the waters. 
