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



simple latticed shell of the same form, only separated from it by a thinner or thicker 

 jelly- veil. The lenticular or discoidal fenestrated shell is therefore an extracapsular or 

 " cortical shell," without an enclosed medullary shell. 



The few genera of the Cenodiscida differ only in the shape of the equatorial margin 

 of the lenticular disk. In the first subfamily, Zonodiscida, the margin is either quite 

 simple (Cenodiscus) or surrounded by a smooth; solid equatorial girdle (Zonodiscus). 

 In the second subfamily, Trochodiscida, the margin is armed with solid radial spines, 

 lying in the equatorial plane. According to the number and disposition of these, marginal 

 spines, we distinguish Stylodiscus (with two spines, opposite in one equatorial axis), 

 Crucidiscus (with four spines, opposite in pairs in two equatorial axes, perpendicular one 

 to another), Theodiscus (with three marginal spines), and Trochodiscus (with numerous, 

 commonly twenty to thirty, irregularly disposed spines). The spines are constantly 

 simple, not branched ; sometimes more conical or cylindrical, at other times more angular 

 or pyramidal. 



The two convex faces of the lenticular shell are constantly of similar shape, commonly 

 smooth, sometimes more or less thorny, or armed with bristle-shaped radial spines. The 

 pores are commonly more or less regular, circular, and disposed in series, which are 

 occasionally more radial, at other times more concentric. If the wall of the hollow lens 

 be rather thick, the difference in the shape of the central and peripheral pores is often 

 striking. The central pores perforating the thick wall perpendicularly are short cylin- 

 drical tubes ; the marginal pores perforating it in an oblique direction are longer conical 

 tubes. The bars between the central pores are often somewhat smaller. 



The Central Capsule of the Cenodiscida is in all cases a perfect, circular, biconvex lens, 

 the equatorial diameter of which is commonly between two-thirds and three-fourths of 

 the enclosing lattice-shell. The interval between the two is filled up by the jelly- veil, 

 or the hyaline " calymma," which is perforated by the numerous pseudopodia that pass 

 through the shell-pores. 



As the Cenodiscida possess the most simple shell-form of all D i s c o i d e a, we may 

 regard Cenodiscus as the common ancestral form of this large section, in the same 

 manner as Cenosphcera is the ancestral form of Sphseroidea, Cenellipsis of the 

 Prunoidea, Cenolarcus of the Larcoidea. But it is also possible that a part of 

 Cenodiscida (or all ?) arises from the Phacodiscida by reduction and loss of the medullary 

 shell. For in some cases we find arising from the inside of the shell centripetal radial 

 beams, which end at a certain equal distance from the hollow centre (PI. 31, fig. 1 1 ; 

 PI. 38, fig. 2). Cenodiscus itself can be derived either from Cenosphcera by com- 

 pression of the spheroidal shell in one axis, or from Sethodiscus by loss of the intra- 

 capsular medullary shell, or from Actidiscus (the lenticular Actissa) by formation of 

 a cortical shell around the lenticular central capsule. 



