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



13. Myelastrum anomalum, n. sp. (PI. 47, fig. 9). 



All four arms of different size and form ; anterior arms broader, posterior longer ; one anterior 

 arm trifid, the three other arms bifid ; length of the branches unequal ; all four angles between the 

 arms unequal. (This anomalous form, seen only once, may be an individual abnormality.) 



Dimensions. Eadius of the arms 0'4 to 0'7 ; constrictions O3. 



Habitat. North Pacific, Station 237, surface. 



Genus 240. Pentalastrum, 1 Haeckel, 1881, Prodromus, p. 461. 



Definition. P orodiscida with five simple, undivided, chambered arms, with- 

 out a patagium. 



The genus Pentalastrum opens the small series of Euchitonida, in which the 

 shell is not provided with three or four arms, as usual, but with five. All forms of 

 this little group are rare. Some species resemble in their external form and in the 

 articulation of their arms certain forms of Asterida. In Pentalastrum, the most simple 

 genus, the five arms are simple, not forked, and without a patagium. It can be derived 

 from Dictyastrum or Stauralastrum by increase of the number of arms. 



Subgenus 1. Pentalastrella, Haeckel. 



Definition. All five arms equal, with equal angles between them. Shell a regular 

 pentagon. 



1. Pentalastrum asteracanthion, n. sp. 



All five arms equal, club-shaped, at their thickened obtuse end three times as broad as at their 

 base, twice as long as broad. Angles between the arms equal. 



Dimensions. Eadius of each arm 0'2, basal breadth 0'03, distal breadth O'OS. 

 Habitat. Pacific, central area, Station 272, depth 2600 fathoms. 



2. Pentalastrum astropecten, n. sp. 



All five arms equal, with five to six distinct, simple joints, the basal joint two-thirds as broad 

 as the terminal joint, which bears a strong conical spine. Angles between the arms equal. 

 (Resembles Pcntinastrum asteriscus, PL 44, fig. 2, but has no patagium.) 



Dimensions. Eadius of each arm 0'14, basal breadth 0'024, distal breadth 0'036. 



Habitat. Pacific, central area, Station 263, depth 2650 fathoms. 



1 Pentakistrum = Little sea-star with five rays ; irtm, *;, 



