86 McLean . — On the Fossil Genus Sporocarpon. 
The Radiolaria form both isospores and heterospores, the latter being 
sexual gametes, and some species even show a complete dimorphism 
according to the type of spore formed. Among the Traquairidae isospores 
are predominant, but specimens frequently show spores of different size 
which may be true heterospores, or may be mere irrelevancies due to 
fungal intrusion during decay. In some cases it is very difficult to accept 
this latter explanation {vide Traquairia in W. 1063). 
One further point of interest presents itself. There is only one class 
of Radiolaria in which no £ zooxanthellae ’ (endobiotic Chrysomonad flagel- 
lates) are present, namely, the Phaeodaria. Among the Phaeodaria we 
find close analogies in form and composition with the skeleton of the 
Traquairidae, but each individual invariably contains a large dark excretory 
a gg re g at i° n - This is quite absent in our fossils, unless the dark mass in 
the capsule of 5 . elegans , PI. VIII, Fig. 3, be so interpreted, so that one can 
scarcely associate the two groups very closely, but at the same time the 
absence of this aggregation of excretory granules raises the interesting 
possibility that endophytic algae may have played a part in the life-history 
of these fossil organisms as in the true Radiolaria. The development of 
elaborate systems of external apertures leaves little doubt that their main 
method of nutrition was, as in the Radiolaria, holozoic, but variation in 
this respect is probably evidenced by the structure of .S. asteroides. The 
skeletal affinities were treated pretty fully in my first paper. I still 
maintain that a chitinous substance seems the most likely material. 
Evidently it was something pretty hard, since the preservation is me- 
ticulously accurate in the smallest details, and it was in most cases 
flexible. 1 Among the Phaeodarian Radiolaria the skeleton often suggests 
the Traquairian form, and it consists of an organic silicate, which is at 
least nearer in nature to the fossils than is the strontium sulphate skeleton 
of the Acantharia. At the same time the internal arrangements in the 
present group are much more nearly those of the Peripylaria. Therefore 
I would suggest that the nearest living analogues of the Traquairidae are 
to be found in the group Orosphaeridae, removed from Phaeodaria to 
Peripylaria by Hacker ( 4 ), which shows a union of the characters of the two 
groups parallel to that observed in the Traquairidae. 2 
Since, as Cavers ( 5 ) has pointed out, the Radiolaria may be derived from 
ancestors like the Gymnodiniaceae, it is not improbable that the Tra- 
1 The Foraminifera possess a chitinous test between the protoplasm and the calcareous test. 
This may develop excessively, not as a consequence of starvation or wandering into brackish water, 
as formerly suggested, but as a variation in any species under any conditions (Heron- Allen, Rep. 
Brit. Ass., Bournemouth, 1919). Wholly chitinous forms may appear as monstrosities. 
2 The Coccolithophoridae (see Murray and Blackman, Nature, lxv. 510, 1897) must also be 
considered as possible descendants, on the evidence of their armour characters, though their cytology 
is not fully known. Indeed, I would be nothing loath to postulate a common plexus of origin for the 
Coccolithophoridae, Traquairidae, Radiolaria, Diatomaceae (as represented by the early liassic genus 
Pyxidicu/a), Prorocentraceae, and Peredineae. 
