medullary shell) a number of radial spines arises, and upon these the secondary shells are formed from within outwards. 165. _Genealogical Tree of the Prunoidea._--The suborder #Prunoidea# is very closely related to the #Sphæroidea#, and is distinguished from it by the elongation of one axis; from the simple lattice-sphere (_Cenosphæra_) is developed a latticed ellipsoid (_Cenellipsis_, Pl. 39, fig. 1). The development of this vertical isopolar main axis is foreshadowed even among the #Sphæroidea#, in that family in which two opposite radial spines grow out of the primary lattice-sphere at the two poles of the vertical main axis (Stylosphærida, Pls. 13, 14). These latter pass over without any sharp boundary into those forms of #Prunoidea# whose ellipsoidal lattice-shell bears two opposite main-spines (Stylatractida, Pls. 15, 16). Other very intimate relationships between the #Sphæroidea# and #Prunoidea# are indicated in certain of the latter by the fact that of the two concentric lattice-shells the inner (medullary) shell is spherical, the outer (cortical) shell ellipsoidal (Pl. 39, figs. 3, 7, 8, 14, 19); often three concentric lattice-shells are present, of which the two inner are spherical intracapsular medullary shells, whilst the outer is an extracapsular cortical shell, ellipsoidal or cylindrical in form (Pl. 39, figs. 4, 12, 17, 18). Owing to the manifold nature of these phylogenetical relations and the variety of their combinations, the derivation of the individual #Prunoidea# from the #Sphæroidea# is rendered very difficult; in addition to which it is possible that the simplest #Prunoidea# (_Cenellipsis_, _Ellipsidium_) have been directly developed from the skeletonless _Actiprunum_ (a form of _Actissa_ with ellipsoidal central capsule, p. 14) by the excretion of a simple ellipsoidal lattice-shell on the surface of their calymma. The phylogeny of the #Prunoidea# is especially complicated by the formation of peculiar transverse constrictions, perpendicular to the longitudinal axis. They are wanting only in the Monoprunida (Ellipsida, Druppulida, and Spongurida); the Dyoprunida (Artiscida and Cyphinida, Pl. 39, figs. 9-19) possess only one such constriction (in the equatorial plane); the Polyprunida, on the other hand, have three, five, or more parallel constrictions (Panartida and Zygartida, Pl. 40). The chambers, which are separated off by these constrictions, may be regarded as polar sections of incomplete cortical shells.