ex THE VOYAGE OF H.M.S. CHALLENGER. 



other axis of the body. The Discoidea are probably polyphyletic, having originated 

 from several different groups of S p h se r o i d e a ; at least two essentially different main 

 groups may be distinguished among them ; of these the one is characterised by the 

 fonnation 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). 



Tlie Phacodiscida (Pis. 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 Dyosphierida or Triosphserida 

 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 (Pis. 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 incom- 

 plete 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 (Pis. 41-46). If those rings from the commencement be 

 interruped by three interradial gaps (gates) the family Pylodiscida arises (PI. 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 (Pis. 46, 47). It is not, however, impossible 

 that part of the Discoidea (especially the Cenodiscida) have originated directly from skeleton- 

 less Collodaria with a lenticular central capsule, such as are found in a subgenus of Adissa 

 {Actidiscus, p. 15). 



167. Genealogical Tree of the Larcoidea. — The suborder Larcoidea presents in 

 the structure, composition, and development of its variously formed lattice-shells much 

 more complicated relations than the other Sphserellaria; it is essentially distinguished 

 from them by the characteristic ground-form of its lattice-shells, which is a " lenteUipsis " 

 or a triaxial ellipsoid (also the ground-form of the rhombic crystallographic system, the 

 rhombic octahedron). Hence all parts of the body are regularly disposed with respect 

 to three different dimensive axes ; all three axes, perpendicular one to another, are 

 isopolar but of different lengths ; the longest is the vertical main axis, the mean the 

 horizontal frontal axis, the shortest the horizontal sagittal axis. In the great majority 

 of the Larcoidea the lenteUiptical ground-form is indicated in the central capsule, 

 even when it is not at once obvious in the skeleton. Since such lenteUiptical central 

 capsules are developed even in Actissa [Actilarcus, p. 16), it is possible that the simplest 

 Larcoidea may have arisen directly from these by deposition of a simple lenteUiptical 

 lattice-shell in the sarcodictyum, on the surface of the calymma (Cenolarcus, PI. 50, 

 fig. 7). It is more pi'obable, however, that these simplest forms [Cenolarcus, Larcarluin) 

 have been developed from the simplest Sphseroidea (CenospAcera), by the spherical 

 body growing unequally in the three dimensions of space. It appears especially likely 



