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



Genus 253. Spongodiscus, 1 Ehrenberg, 1854, Monatsber. d. k. preuss. 

 Akad. d. Wiss. Berlin, p. 237. 



Definition. S pongodiscida with a simple circular disk, without radial 

 appendages and without an equatorial girdle on the margin. 



The genus Spongodiscus represents the most simple and primitive form of the 

 Spongodiscida, or of those Discoidea in which the central disk is more or less spongy, 

 composed of an irregular fine framework. In my Monograph (1862, pp. 452, 460, 469) 

 I had separated the true Spongodiscus (first described by Ehrenberg, loc. cit.) and the 

 Spongocydia; the former being characterised by the irregular spongy framework of the 

 whole disk, whilst in the latter this framework includes in the central part some con- 

 centric circular rings (approaching Porodiscus). In the same way afterwards Stohr (1880, 

 loc. cit.J separated the genus Spongospira as spongy disks, which include in the central part 

 some spiral convolutions. But as these differences are rather inconstant and not sharply 

 discernible, I think it now better to regard these three forms as subgenera of Spongodiscus. 

 All three have the common simple circular disk, without any marginal appendages. 



Subgenus 1. Spongodiscidus, Haeckel. 



Definition. Spongy framework of the disk quite irregular, without concentric rings 

 or spiral convolutions. 



1. Spongodiscus mediterraneus, Haeckel. 



Spongodiscus mediterraneus, Haeckel, 1862, Monogr. d. Radiol., p. 461, Taf. xii. figs. 14, 15. 



Spongy disk plain on both sides, with quite an irregular framework, without concentric rings and 

 without radial piercing beams. Texture everywhere uniform ; meshes eight to ten times as broad 

 as the bars. 



Dimensions. Diameter of the disk 0'08 to 0'24, of the meshes 0'005 to O008. 



Habitat. Mediterranean (Messina), Haeckel ; also fossil in the Tertiary rocks of Sicily 

 (Grotte), Stohr. 



2. Spongodiscus radiatus, n. sp. 



Spongy disk plain on both sides, with quite irregular framework, without concentric rings, but 

 with numerous piercing radial beams which are not prolonged into marginal spines. Texture every- 

 where uniform ; meshes once and a half to twice as broad as the bars. 



Dimensions. Diameter of the disk 012 to 015, of the meshes 0'002 to 0'003. 



Habitat. Pacific, central area, Station 268, depth 2900 fathoms. 



1 Spongodiscus = Spongy disk ; 



