REPORT ON THE RADIOLARIA. 287 



more ; all the chambers form a single series and have a common main axis. All 

 constrictions lie in planes parallel to the equatorial plane of the original ellipsoid ; 

 in the centre of the latter constantly lies a double " medullary shell," composed of 

 two concentric, either spherical or lenticular, compressed shells. In all Panartida we 

 call the two inner chambers (on both sides of the equatorial constriction) " proximal 

 chambers," the two outer chambers (on the poles of the main axis) " distal chambers." 

 The four-chambered cortical shell of the Panartida is either simple (in Panartus, 

 PI. 40, figs. 14) or double, with an external mantle (as in Peripanartus, PI. 40, 

 figs. 57). The simplest form of the subfamily is Panartus (loc. cit.). In this 

 case also on both poles of the main axis may be developed solid spines, or hollow 

 fenestrated tubes (Panarium, PL 40, fig. 9). 



The seventh and last family of the Prunoidea, the Zygartida, is most nearly 

 allied to the Panartida, and appears as a further developmental step from that family. 

 Whilst in the Panartida the cortical shell is constantly four-chambered, with three 

 parallel ring-like constrictions, in the Zygartida it is always prolonged and composed 

 of six or more chambers, separated by five or more ring-shaped constrictions, in the 

 middle of which is the equatorial stricture. In the centre of the latter (as also in the 

 Panartida) always lies the double medullary shell, composed of two concentric, spherical, 

 or lenticular shells. The number of the chambers of the cortical shells is commonly 

 six or eight (with five to seven ring strictures), but it often mounts to ten and some- 

 times to twenty (with nineteen strictures), as in some species of Zygartus (PL 40, fig. 13). 

 All the chambers lie in one series, one behind another, with a common main axis. The 

 cortical shell is usually simple (in Ommatocampe, PL 40, fig. 10), sometimes double 

 (in Desmocampe, PL 40, fig. 12), rarely triple (in Zygocampe, PL 40, fig. 13). In 

 all three cases hollow fenestrated tubes may be developed on the poles of the main axis. 



The morphological references and the phylogenetic affinities of all Prunoidea 

 are so complex, that they seem to represent a quite natural group ; all forms of 

 it may be derived from the common ancestral form Cenellipsis. But a far more 

 difficult question is the manner in which its pedigree may be constructed. The 

 oldest family is probably the simplest, namely, Ellipsida. From this the Druppulida 

 may be derived by production of medullary shells, the Artiscida by equatorial 

 constriction. The Cyphinida can be produced either from the Druppulida by 

 equatorial constriction or from the Artiscida by development of medullary shells. 

 The Panartida appear as further developmental steps of the Cyphinida, by dupli- 

 cation of the chamber number ; and the Zygartida as further productions of the 

 Panartida, by increasing the number of the chambers. 



The seven subfamilies of the Prunoidea can be arranged in two sections accord- 

 ing to the presence or absence of medullary shells. The Ellipsida, Spongellipsida, and 

 Artiscida possess a simple cortical shell, without a medullary shell ; they represent the 



