REPORT ON THE RADIOLARIA. 407 



or four spongy, radial arms the former as well as the latter lying in the equatorial plane 

 of the disk, either regularly or irregularly disposed. 



The Equatorial Margin of the lenticular disk exhibits in all six families of 

 Discoidea similar characters, mainly serving for the distinction of subfamilies and 

 genera. In the most primitive genera of all six families the margin is simple, without 

 radial prolongations (spines or arms) ;' it is quite simple in Cenodiscus, SetTiodis- 

 cus, Phacodiscus, Lithocyclia, Coccodiscus, Archidiscus, Porodiscus, Pylodiscus, and 

 Spongodiscus. In some genera the simple margin of the lenticular disk is bordered 

 and surrounded by a thin, hyaline, equatorial girdle of silex, either quite solid or 

 slightly porous (Zonodiscus, Periphcena, Perizona, Perichlamydium, and Spongophacus). 



A quite peculiar and remarkable character of few genera is the development of one or 

 two oscula, larger marginal openings, which are surrounded by a corona of spines, and 

 probably are fit for the issue of a peculiar bunch of pseudopodia or of a " sarcode flagellum." 

 Such oscula occur only in two families of Cyclodiscaria; in the Porodiscida and 

 Pylodiscida ; in the former Ommatodiscus, in the latter Discopyle (PL 48, figs. 19, 

 20) is distinguished by a single marginal osculum ; besides this, in the former occurs 

 Stomatodiscus, with two such oscula, opposite on the poles of one axis (PI. 48, fig. 8). 

 These oseula may be compared with the similar polar formations in some Ellipsida 

 (Lithomespilus) and in many Cyrtoidea ; but they do not prove a nearer affinity with the 

 latter, and are only analogous, not homologous. 



Radial Spines occur on the margin in the equatorial plane of the Discoidea in the 

 greatest variety of number, form, size, and disposition. If the number be low (between 

 two and eight) they are commonly regularly disposed ; if the number be larger (ten to 

 twenty or more) their disposition becomes commonly more or less irregular. The 

 regular disposition is of great promorphological importance, as indicating the axes in 

 which the growth is preponderant, and introduces other peculiar radial formations. 

 Regarding these axes we can generally distinguish two groups, Artiacantha with a 

 paired number (two, four, eight), and Perisaacantha with odd numbers (usually three). 

 The section of Artiacantha could be divided into the three following groups: A. 

 Stylodiscida, with two radial spines only, lying opposite on both poles of one equatorial 

 axis (the " first cross axis ") Stylodiscus, Sethostylus, Stylocyclia, Xiphodictya, 

 Spongolonche (PI. 31, figs. 9-12; PL 38, fig. 1; PL 42, figs. 10-12, &c.) ; 

 B. Staurodiscida, with four radial spines, lying opposite in pairs on the poles of two 

 crossed equatorial axes, perpendicular to one another (first and second cross axes) 

 Crucidiscus, Sethostaurus, Staurocyclia, Staurodictya, Spongostaurus (PL 31, figs. 1-8 ; 

 PL 37, figs. 1-4 ; PL 42, figs. 1-6 ; PL 48, fig. 2, &c.) ; C. Octostylida, with eight 

 radial spines, opposite in pairs in four axes, which are crossed at angles of 45- 

 Heliosestrum, Astrosestrum, &c. (PL 32, figs. 4, 5 ; PL 34, figs. 3, 6) ; in this latter case 

 sometimes the radial symmetry is the same as in many Medusae, four larger (perradial) 



