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



Genus Benthaster, Sladen. 

 Benthaster, Sladen, Journ. Linn. Soc. Lond. (Zool.), 1882, voL xvi. p. 242. 



Form depressed. Marginal contour stellato-pentagonoid. Abactinal area convex. 

 Actinal area plane. 



Supradorsal canopy rudimentary. No muscular fibrous bands. No spiracula. Nida- 

 mental cavity more or less aborted. Paxillse with subfascicular crowns ; spinelets tri- 

 laminate, of extraordinary length, delicacy, and number, protruding the greater portion 

 of their length naked beyond the supradorsal membraue. Paxillae probably devoid of 

 investing membrane. Papulae simple, globular, sessile sacs, comparatively large. Special 

 dorsolateral (1 supero-marginal) plates at the extremities of the rays. Ossicles of the 

 abactinal surface cruciform, greatly attenuated, the whole calcareous framework being 

 reduced to a minimum. 



Adambulacral armature consisting of one or two spinelets, which are long, needle- 

 shaped, independent, and not united by membrane. Aperture-papillae more or less 

 modified. Segmental apertures (?) aborted. 



Mouth-plates of the Hymenaster type. Two pairs of secondary or superficial mouth- 

 spines, robust, clavate, thorny, probably without saccular membrane. Mouth-spines proper 

 two or three, the innermost resembling the anterior pair of secondaries, only rather 

 smaller, the others pointed. 



Actino-lateral spines merged in the actinal floor. 



Remarks. — The superficial aspect of this genus recalls, to a certain extent, that of 

 Korethraster, from which, however, it is structurally widely separate. The rudimentary 

 supradorsal membrane, and the long, fasciculated, naked, paxillar spinelets, protruding 

 far beyond the membrane, readily distinguish the genus from Hymenaster ; whilst the 

 simple independent armature of the adambulacral plates, not forming combs and not 

 webbed, at once distinguishes Benthaster from Marsipaster, the only other form with 

 which it can be compared. 



Clwrology of the Genus Benthaster. 

 a. Geographical distribution : — 



Pacific : Two species between the parallels of 40° N. and 10 S. 



Benthaster wyville-thomsoni, from the Mid-North Pacific, near the 

 meridian of 170° E. Benthaster penicillatus, off the north coast of 

 New Guinea, south-west of the Admiralty Islands. 



p. Bathymetrical range: 1070 to 2900 fathoms. 



y. Nature of the Sea-bottom : Benthaster wyville-thomsoni on Red clay ; Benthaster 

 penicillatus on Blue mud. 



