166. _Genealogical Tree of the Discoidea._--The suborder #Discoidea# is closely related to the #Sphæroidea#, but separated from it by shortening of one axis; from a simple lattice-sphere (_Cenosphæra_) a latticed lens or flattened spheroid is developed, whose circular equatorial plane is larger than any other section (_Cenodiscus_, Pl. 48, fig. 1). The formation of this horizontal equatorial plane is perhaps indicated in that family of #Sphæroidea# in which four crossed radial spines, lying in one plane, are developed (Staurosphærida, Pls. 15, 31, 42). The morphological and phylogenetical relations of the #Discoidea# to the #Sphæroidea# are precisely the converse of those of the #Prunoidea#; in the latter the vertical axis appears longer, in the former shorter than any {cx}other axis of the body. The #Discoidea# are probably polyphyletic, having originated from several different groups of #Sphæroidea#; at least two essentially different main groups may be distinguished among them; of these the one is characterised by the formation of a large extracapsular lenticular cortical shell (Phacodiscaria), whilst in the other this typical "Phacoid shell" or lattice-lens is wanting (Cyclodiscaria, compare pp. 403-409). The Phacodiscida (Pls. 31-35) perhaps constitute the primitive group of the Phacodiscaria, their lenticular or Phacoid cortical shell being connected by radial bars with one or two concentric spherical medullary shells; they may have originated directly from the Dyosphærida or Triosphærida by flattening of the spheroidal cortical shell. From the Phacodiscida the Cenodiscida (if indeed they be not the primitive stem-form) have been developed by retrogression and loss of those medullary shells. The Coccodiscida (Pls. 36-38), on the other hand, have been developed from the Phacodiscida by the addition of concentric rings of chambers, which may be regarded as incomplete cortical shells, only the equatorial portion of which is developed. Perhaps the Porodiscida, the primitive group of the Cyclodiscaria, have arisen in a similar way; they lack, however, the typical Phacoid shell, the concentric rings of chambers being directly applied to a small spherical medullary shell in the equatorial plane (Pls. 41-46). If those rings from the commencement be interrupted by three interradial gaps (gates) the family Pylodiscida arises (Pl. 38, figs. 6-20). If, on the contrary, the concentric radially divided chambers of the Porodiscida become quite irregular and spongy, they pass over into the Spongodiscida (Pls. 46, 47). It is not, however, impossible that part of the #Discoidea# (especially the Cenodiscida) have originated directly from skeletonless #Collodaria# with a lenticular central capsule, such as are found in a subgenus of _Actissa_ (_Actidiscus_, p. 15).