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



Genus 253. Sjwngodiscus,^ Ehreuberg, 1854, Monatsber. d. k. preuss. 

 Akad. d. Wiss. Berlin, p. 2-37. 



Definition. — S p o n g o d i s c i d a with a simple circular disk, witliout 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 D i s c o i d e a 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 

 Sjyongocyclia; 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.) separated the genus Spongosjnra as spongy disks, Avhich include in the central part 

 some spiral convolutions. But as these difli"erences are rather inconstant and not sharply 

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

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



Subgenus 1. Spongodiscidus, Haeekel. 



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

 or spiral convolutions. 



1. Spongodiscus med iter ran ens, Haeekel. 



Spongodiscus mediterraiieus, Haeekel, 1862, Jlonogr. d. Eadiol., p. 461, Taf. sii. figs. 11, 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. 



DimensioTis. — Diameter of the disk 0-08 to 0-24, of the meshes O'OOS to 0-008. 



Habitat. — Mediterranean (Messina), Haeekel ; also fossil in tlie Tertiary rocks of Sicily 

 (Grotte), Stohr. 



2. Spongodiscus radiatus, n. sp. 



Spongy disk plain on both sides, with quite irregular framework, without concentric rmgs, 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 0'12 to 0"15, of the meshes 0'002 to 0'003. 



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



• /Spon(7odisc«« = Spongy disk; (s7:nyyo;, I/itko; 



