197. _Phæosphæria with a Sphæroid Skeleton._--The order #Phæosphæria# includes those PHÆODARIA which possess a spherical (sometimes slightly modified) lattice-shell without the characteristic aperture of the #Phæogromia#. They have probably arisen independently of these, though they may have been derived from the Castanellida by loss of the shell-aperture, which was present originally. The four families which we have distinguished among the #Phæosphæria#, are so different in the structure of their lattice-shell that their phylogenetic connection is doubtful. In the Orosphærida (Pls. 106, 107) and the Sagosphærida (Pl. 108) the whole lattice-shell consists of a single piece and is unjointed (without astral septa); in the former it is very firm and massive, with thick laminated trabeculæ and polygonal meshes; in the latter it is very delicate and brittle, with filiform trabeculæ and large { cxxvi}triangular meshes. On the other hand, the voluminous shell of the Aulosphærida (Pls. 109-111), and of the Cannosphærida (Pl. 112), is characterised by a very peculiar system of joints; it is composed of numerous separate cylindrical tubes, which are placed tangentially and united at the nodes by stellate partitions or astral septa. The Cannosphærida possess further a simple central Cyrtoid shell, connected with the outer jointed shell by hollow radial trabeculæ. Since many Aulosphærida possess rudiments of such centripetal trabeculæ it is possible that these latter have been derived from the former by the loss of the central Cyrtoid shell; the formation of this monaxon shell perhaps indicates descent from the #Phæogromia# (Castanellida).
198. _Phæogromia with a Cyrtoid Skeleton._--That order of the PHÆODARIA which we designate #Phæogromia#, contains many very different forms, all agreeing in the possession of a Cyrtoid skeleton, or a monaxon lattice-shell, which has a large aperture at one pole of its vertical main axis (§ 123). This Cyrtoid skeleton is sometimes ovoid or conical, sometimes lentiform or helmet-shaped, sometimes polyhedral or almost spherical. Although the principle of its structure is simple and often like that of the Monocyrtida among the NASSELLARIA, yet the structure of the wall and of the apophyses is so different in the various groups of the #Phæogromia#, that the order is probably polyphyletic, and its Cyrtoid shells have arisen independently of each other. Only in the Castanellida (Pl. 113) does the shell-wall usually consist of simple lattice-work; in the Challengerida, on the other hand (Pl. 99), it has an extremely fine Diatom-like structure; in the Medusettida (Pls. 118-128) a peculiar alveolar structure, and in the Circoporida (Pls. 114-117) and Tuscarorida (Pl. 100) it possesses a characteristic porcellanous constitution (with tangential spicules in a porous cement-mass); in the latter of these groups the surface is smooth, in the former peculiarly tabulate; the two families have also different stem-forms.