552 THE VOYAGE OF H.M.S. CHALLENGE!?. 



7. Dicranastrum trifarium, n. sp. 



Arms trifid, three times as long as broad at their base ; each arm with three pointed terminal 

 branches, ending in strong sulcate spines ; the middle branch twice as large as the two others. The 

 simple basal part of each arm two and a half times as long as the trifid distal part. 



Dimensions. Eadius of the arm 0'35, basal breadth 0'03, greatest breadth (in the distal part) 

 0-05. 



Habitat. South Pacific, Station 298, surface. 



Subgenus 3. Tetracranastrum, Haeckel. 



Definition. Each cross-arm with four terminal branches, the two fork-branches 

 being again bifurcated. 



8. Dicranastrum bifurcatum, n. sp. (PI. 47, figs. 1, la). 



Arms doubly forked or quadripartite, six times as long as broad at their base ; each arm in its 

 proximal half simple, three times as long as broad ; in its distal half doubly forked ; the secondary 

 branches with blunt, roundish ends, nearly as large as the primary branches. Central disk (fig. la) 

 with three concentric rings around the central chamber ; from its periphery radiate thin radial 

 beams in the spongy framework of the delicate arms. The central capsule has the same form as 

 the skeleton, and is only a little smaller. 



Dimensions. Eadius of the arm 045, basal breadth 0'08 ; breadth of the terminal branches 

 0-03. 



Habitat. North Pacific, east of Japan, Station 241, surface. 



Genus 239. Myelastnim, 1 Haeckel, 1881, Prodromus, p. 460. 



Definition. P orodiscida with four forked, spongy, or chambered arms, with- 

 out a patagium ; shell bilateral, with paired different arms ; two equal anterior arms of 

 different shape from the two equal posterior arms. 



The genus Mydastrum differs from the foregoing Dicranastrum, the ancestral 

 form, by the twofold differentiation of the four arms. Whilst in the latter all four arms 

 are equal, separated by equal angles, here the two anterior arms are constantly different 

 from the two posterior. The lateral angles between the two arm-pairs are equal, while 

 the posterior and the anterior angle (between the two arms of each pair) are more or 

 less different. The shell assumes, therefore, a very characteristic bilateral form, similar 

 to the " quadricorn cross " of the grey central substance in the transverse section of the 

 human medulla spinalis. Though the spongy shell is commonly a most delicate and 



1 Mye'astn m - Medullary starrulet ; 



