REPORT ON THE RADIOLARIA. c ix 



lattice-shell (primary 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 Sphaeroidea, and is distinguished from it by the elongation of one axis; 

 from the simple lattice-sphere (Cenosphcera) is developed a latticed ellipsoid (Cenellipsis, 

 PI. 39, fig. 1). The development of this vertical isopolar main axis is foreshadowed 

 even among the Sphaeroidea, 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 

 (Stylosphaerida, Pis. 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, Pis. 15, 16). Other very intimate relationships between the 

 Sphaeroidea 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 (PI. 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 (PI. 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 

 Sphaeroidea 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, 

 PI. 40). The chambers, which are separated off by these constrictions, may be regarded as polar 

 sections of incomplete cortical shells. 



166. Genealogical Tree of the Discoidea. The suborder Discoideais closely related 

 to the Sphaeroidea, but separated from it by shortening of one axis ; from a simple 

 lattice-sphere (Cenosphcera) a latticed lens or flattened spheroid is developed, whose 

 circular equatorial plane is larger than any other section (Cenodiscus, PI. 48, fig. 1). 

 The formation of this horizontal equatorial plane is perhaps indicated in that family of 

 Sphaeroidea in which four crossed radial spines, lying in one plane, are developed 

 (Staurosphserida, Pis. 15, 31, 42). The morphological and phylogenetical relations of 

 the Discoidea to the Sphaeroidea are precisely the converse of those of the 

 Prunoidea; in the latter the vertical axis appears longer, in the former shorter than any 



