4G2 THE VOYAGE OF H.M.S. CHALLENGER. 



Subfamily Korethrasterin^e, Sladen, 1888. 

 Genus Korethraster, Wyville Thomson. 

 Korethraster, Wyville Thomson, The Depths of the Ssa, London, 1873, p. 120. 



Form stellate. Kays moderately produced. Abactinal surface convex and more or 

 less arched, meeting the actinal surface, which is plane, with an abrupt angle. Inter- 

 brachial arcs with a lunate outline. 



Abactinal surface entirely covered with comparatively large, thin, squamous plates 

 (oval or subcircular in the type form), which imbricate more or less upon one another. 

 Each plate bears a compact fasciculus of elongate spinelets ankylosed together at their 

 base, and articulated on a mammillate boss on the plate. No papulas are present on the 

 abactinal surface. 1 A well-defined channel traverses the median interradial line. 



A single series of short but broad band-like plates (? infero- marginals) occupies the 

 outer half of the actinal surface of the rays and defines the margin. Each plate bears on 

 its free extremity in the margin a flattened, scoop-like, or flaring lateral spine. 



Adambulacral plates short but broad, equal in length and correspondent to the 

 infero-marginal plates ; and appearing to form with these, at least on the inner half of 

 the ray, a single transverse band passing from the ambulacral furrow to the margin, 2 

 the bands being separated by a distinct interspace of nearly equal breadth. Armature 

 consisting of a single transverse series of rather robust, slightly flaring spines (four in 

 number), rather widely spaced, one at each extremity, the innermost or furrow spine 

 rather smaller and directed over the furrow, the outermost radiating outward ; the series 

 of these spines form longitudinal rows along the ray. 



No actinal intermediate plates are present. 



Mouth-plates with a prominent median keel resembling more or less closely the same 

 structure in Hymenaster. Armature consisting of: (1.) A marginal series of four spines 

 on each plate, the innermost largest and directed horizontally. (2.) On the actinal surface 

 of each plate a large robust secondary mouth-spine, placed near the adoral end of the 

 plate. 



Madreporiform body small, circular, situated near the centre of the disk, marked with 

 very coarse deep striae. 



1 I believe that a single longitudinal series of papular orifices is present on the actinal surface on each 

 side of the ambulacral furrow between the adambulacral plates, situated in the dark shade shown in the figure 

 9 on PL LXXX., but I am unable to make the statement definitely without dissection, which is of course 

 undesirable in the unique specimen at my disposal. 



2 The junction of the plates is probably ankylosed, and is masked by the outermost spine of the adam- 

 bulacral armature. This has probably led previous writers to speak of the actinal skeleton of Korethraster as 

 formed of single band-like plates passing from the furrow to the margin. On the outer part of the ray near 

 the extremity, the direction of the adambulacral and infero-marginal plates is not in the same line, and the 

 two series of plates may be better distinguished there. 



