102. _The Chemical Peculiarities of the Skeleton._--The chemical composition of the skeleton shows very marked variations in the different legions of the Radiolaria. The two legions SPUMELLARIA and NASSELLARIA (united formerly as "Polycystina") form their skeleton of pure silica (see note A, below); the legion PHÆODARIA of a silicate of carbon (see note B), and the ACANTHARIA of a peculiar organic substance--acanthin (see note C). This explains the well-known fact that the deposits of fossil Radiolaria (or Polycystine marls) are composed exclusively of the skeletons of SPUMELLARIA and NASSELLARIA, those of the ACANTHARIA and PHÆODARIA being entirely absent (in the case of the last group, however, exception must be made in favour of the Dictyochida, or those PHÆODARIA {lxix}whose skeleton is made up of isolated scattered tangential siliceous fragments). The enormous deposits of Radiolarian skeletons in the deep sea of today, which constitute the Radiolarian ooze, consist, like the fossil Polycystine marls, almost exclusively of the shells of SPUMELLARIA and NASSELLARIA, though here the acanthin skeletons of the ACANTHARIA may be present in very small numbers, and the silicate skeletons of the PHÆODARIA, which offer more resistance to the solvent action of sea-water, somewhat more abundantly. Calcareous skeletons do not occur in the Radiolaria (see note D). A. The pure siliceous skeletons of the Polycystina were first recognised in 1833 by Ehrenberg in chalky marls (L. N. 2, p. 117). Since the two legions ACANTHARIA and PHÆODARIA were entirely unknown to Ehrenberg, his name Polycystina has reference only to the SPUMELLARIA and NASSELLARIA.