404 THE VOYAGE OF H.M.S. CHALLENGEE. 



The Cenodiscida (PI. 48, figs. 1-3) open the series of the Discoid ea as their 

 simplest forms ; a discoidal or lenticular simple lattice-shell encloses a central capsule of 

 the same form, and is separated from it by the calymma or jelly-veil. The common 

 ancestral form of this family is Cenodiscus, without radial marginal spines ; it can be 

 derived from Cenosphcera in the most simple way, by flattening in one axis. If on the 

 equatorial margin of the lens a peculiar solid girdle be developed, we obtain Zonodiscus ; 

 in all other genera of the Cenodiscida radial s^sines are developed on the margin. As the 

 simple lenticular cortical shell of Cenodiscus, in which the central capsule is enclosed, is 

 most characteristic not only of this family, but also of the two following families, we call 

 it the phacoid shell (that is, a lenticular extracapsular or cortical lattice-shell). 



The P h a c o d i s c i d a (Pis. 31-35), the second family, have the same extra- 

 capsular " phacoid shell " as the Cenodiscida, but differ from these by the possession of one 

 or two intracapsular concentric medullary shells, which are connected with the former by 

 radial beams, perforating the lenticular central capsule. The radial beams are commonly 

 numerous, and arranged in two opposite bunches around the shortened main axis. But 

 often also besides these occur other longer radial beams, situated in the equatorial plane ; 

 the number of these is commonly four, and they form a regular rectangular cross, lying- 

 opposite in pairs in two equatorial diameters, perpendicular one to another. In 

 the simplest genera of this family (the Sethodiscidct) the equatorial margin of the 

 phacoid shell is simple or surrounded by a solid smooth girdle ; in all other genera are 

 developed on the margin solid radial spines lying in the ecjuatorial plane, either regularly 

 disposed in a somewhat constant number (two to eight, Heliosestrida), or irregularlj? dis- 

 posed, in a larger and more variable number (ten to twenty or more, Heliodiscida). 



The Coccodiscida (Pis. 36-38) form a third family of the D i s c o i d e a, 

 dii'ectly associated with the Phacodiscida ; both have the same characteristic extra- 

 capsular " phacoid shell," which is connected by radial beams with a simple or double, 

 intracapsular medullary shell. But whilst in the foregoing family the equatorial margin 

 of the phacoid shell is simple or only armed with radial spines, in the Coccodiscida it 

 is surrounded by peculiar concentric chambered girdles, or rings, which resemble those 

 of the following family, the Porodiscida. Each of these " chambered girdles " is 

 composed of a circular ring in the equatorial plane, a variable number of radial beams 

 dividing it into incomplete chambers, and two porous cover-plates or " sieve-plates," 

 covering the upper and lower face of the disk. These sieve-plates may be regarded as 

 incomplete lenticular cortical shells, which are only developed in the peripheral part of 

 the disk, whilst their central part is represented by the only complete cortical shell, the 

 " phacoid shell." The number of these concentric chamber-girdles amounts to from one to 

 ten or more. The margin of the disk is either simple (Lithocyclida) or armed with 

 radial spines (Stylocyclida), or provided with two to five chambered radial arms 

 (Astracturida) ; the structure of the arms is the same as that of the girdles. 



