550 THE VOYAGE OP H.M.S. CHALLENGER. 



thickened, club-shaped, with a pyramidal terminal spine. The ends of the arms are connected by 

 a riband-shaped, straight, spongy patagium of the same breadth as the arms. Between the rhom- 

 boidal patagium and the arms remain four large rectangular triangles as interbrachial openings.' 



Dimensions. Eadius of the longer arms 0'2, of the shorter 0'15 ; basal breadth 0'02 ; length of 

 the sides of the rhombic patagium 0'25. 



Habitat. Fossil in the rocks of Barbados. 



Genus 238. Dicranastrum, 1 Haeckel, 1881, Prodromus, p. 460. 



Definition. P orodiscida with four forked, spongy, or chambered arms, without 

 a patagium ; shell regular (not bilateral), with four equal arms crossed at right 

 angles. 



The genus Dicranastrum comprises a number of very remarkable, hitherto unknown, 

 Euchitonida, which are rather common in the Pacific (mainly on the surface), and 

 characterised by the bifurcation of the cross-arms of the regular square shell. It bears 

 therefore to its probable ancestral form, Stauralastrum, the same relation that in the 

 triradiate Euchitonida Chitonastrum does to Dictyastrum. The arms are commonly 

 of very delicate structure, more or less spongy. 



Subgenus 1. Dicranaster, Haeckel. 

 Definition. Each cross-arm with two simple branches. 



1. Dicranastrum furcatum, n. sp. (PI. 47, fig. 2). 



Arms simply forked, three times as long as broad at their base, with eight blunt ends of the 

 fork-branches. The simple proximal half of each arm about the same size as each branch of the 

 dichotomous distal part, twice as long as broad. Edges of the arms ragged. 



Dimensions. Eadius of each arm 0'38, basal breadth 012 ; breadth of the forked part 0'3. 



Habitat. Pacific, central area, Station 271, surface. 



2. Dicranastrum dichotomum, n. sp. 



Arms simply forked, four times as long as broad at their base ; each arm with two blunt branches. 

 The simple proximal part of each arm is three times as long and twice as broad as each branch of the 

 dichotomous distal part. Ends of the arms blunt, truncated. 



Dimensions. Eadius of each arm O35, basal breadth 0'08 ; breadth of the forked part 0-2. 



Habitat. South Pacific, Station 281, surface. 



1 Dicranastrum = Fork-starrulet ; 



