Sigma (a slender rod-like spicule curved in the shape of the letter C), [220], [222]
Sigmaspire (a slender rod-like spicule twisted through about a single revolution of a spiral, and consequently having the form of a C or an S according to the direction in which it is viewed), [222]
Siliceous plates, of test of Rhizopods, [29], [53] f.
Siliceous skeleton of Heliozoa, [71], [74];
of Radiolaria, [76] f.;
of Silicoflagellata, [110];
of Sponges, [171] f., [175] f., [195] f.
Silicified cell-wall of some Dinoflagellates, [130]