109. _The Skeleton of the Spumellaria._--The skeletons of the SPUMELLARIA or PERIPYLEA consist of silica, and are very different and of independent origin in the two orders of this legion. The first order, #Collodaria#, have either no skeleton whatever (#Colloidea#, p. 10, Pls. 1, 3), or their skeleton is _Beloid_, a loose extracapsular envelope of spicules, consisting of numerous unconnected portions; the separate parts are usually disposed tangentially, either as simple or compound siliceous spicules (#Beloidea#, p. 28, Pls. 2, 4). The second order of SPUMELLARIA, on the other hand (#Sphærellaria#, p. 49), develops a siliceous lattice-shell which consists of a single piece, and is remarkable for the extraordinary variety of its forms (pp. 50-715, Pls. 5-50). To this order belong not less than three hundred genera and seventeen hundred species of the Challenger Radiolaria (that is, about two-fifths of all the genera and species). In spite of this extreme richness in different forms this large group must be regarded as _monophyletic_, since all its forms may be quite naturally derived from a common stem-form, a _simple lattice-sphere_ (_Cenosphæra_, p. 61, Pl. 2). The twenty-eight families of #Sphærellaria# may be distributed in four suborders, among which the #Sphæroidea# constitute the {lxxiv}stem-forms, since they retain the original spherical shape (Pls. 5-8, 11-30). In the other three suborders a vertical main axis is developed, which in #Prunoidea# is longer, in #Discoidea# shorter than the other axes of the shell. Hence the shell of the #Prunoidea# (p. 284, Pls. 13, _bis_, 17, 39, 40) is ellipsoidal or cylindrical, that of the #Discoidea#, on the other hand, lenticular or discoidal (p. 402, Pls. 31-38, 41-48). Finally, the shell of the fourth suborder, #Larcoidea#, is lentelliptical; it has the ground-form of a triaxial ellipsoid, and is characterised by the possession of three unequal dimensive axes, or three isopolar axes of different lengths perpendicular to each other (p. 599, Pls. 9, 10, 49, 50).