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



girdle, which is about half as broad as the radius of the disk. Twelve to sixteen bristle-shaped 

 radial spines, irregularly disposed, are connected by the girdle near to the points. 



Dimensions. Diameter of the disk (with six rings) 012, with the girdle 0'2 ; breadth of each 

 ring O'Oll ; pores in the central disk 0'004, in the girdle 0'002. 



Habitat. Fossil in Tertiary rocks of Sicily, Grotte, Stohr. 



6. Stylochlamydium spongiosum, Haeckel. 



PeriMamydium spongiosum, Stohr, 1880, Palseontogr., vol. xxvi. p. 109, Taf. v. fig. 3. 



Kings of the disk partly concentric, partly spiral, more or less irregular and often interrupted, 

 with increasing breadth from the centre. Central part of the disk more or less spongy and 

 obscure. Equatorial girdle half as broad as the radius of the chambered disk, with smaller pores 

 than the latter, pierced by twenty to thirty thin, bristle-shaped radial beams, which proceed over 

 the margin of the disk. 



Dimensions. Diameter of the disk (with ten rings) 0'2, with 'the girdle 0'3 ; breadth of the 

 rings 0-005 to 0'015 ; pores O'OOl to O'OOo. 



Habitat. Pacific, central area, Station 268, depth 2900 fathoms ; also fossil in the Tertiary 

 rocks of Barbados and Sicily. 



Subfamily 5. EUCHITONIDA, Haeckel. 



Definition. P orodiscida with two or more (commonly three or four) radial 

 chambered or spongy arms on the margin of the concentrically amrulated disk, situated 

 in its equatorial plane (with or without a connecting patagium between the arms). 



Genus 223. Amphibrachium, 1 Haeckel, 1881, Proclromus, p. 460. 



Definition. Porodiscida with two simple, undivided, chambered arms, 

 opposite in one axis, without a patagium. 



The genus Amphibrachium opens the long series of the Euchitonida, or of those 

 Porodiscida which bear on the margin of the circular central disk a certain number of 

 chambered arms, composed of a series of chambers which are separated by transverse 

 .septa. The first group or tribe of this subfamily is formed by the Amphibrachida, 

 in which the disk bears only two arms opposite on the poles of one axis. The simplest 

 form of these is Amphibrachium, in which both arms are simple, equal, and without a 

 patagium or spongy connecticulum. 



1 Amphilraehium = Shell with two arms ; <*/*?>/, 



