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



and bombous over the disk, tapering off rather steeply to the extremity of the rays. 

 Actinal surface slightly concave. 



The supradorsal membrane is marked out with conspicuous reticulated lines of mem- 

 brane, although no regular divisional fibres or independent tendinous network can be made 

 out — the fibres present being fine and only distinguishable when highly magnified. The 

 paxillfe are moderatelv long, with the spinelets radiating well outward, about six to eight 

 or even more in number ; the corrugated membrane which they support lying thick and 

 baggy over their tips, and forming the regular lines and cross-lines whereby the character 

 above noted is produced. None of the spinelets are more prominent than the rest ; and, 

 although a central one appears to be normally present in the centre of the mesh, this is 

 very frequently absent towards the end of the rays. The membrane is semitransparent, 

 and contains no spicules ; six to ten large-sized, irregularly disposed spiracula occur in 

 each mesh-like area. 



The ambulacral furrows are moderately wide and sublanceolate ; the tube-feet more 

 or less alternate, and show distinctly a tendency towards quadruple arrangement. The 

 armature of the adambulacral plates consists of moderately long and slender spinelets, 

 each comb having three or four (and occasionally five) ; the innermost spinelet is not 

 more than one-third of the size of the next, and is placed in advance of, or aboral to, the 

 rest of the comb ; the outer spinelet is usually rather longer than the others. The web- 

 membrane is'semitransparent, moderately indented, and after passing from the outermost 

 spinelet of the comb, is attached to the web of the actino-lateral spine slightly in advance 

 of the actino-lateral spine belonging to its own adambulacral plate — the spinelet hanging 

 right over the terminal piece of web, which does not reach far out as in Pteraster ruga- 

 tus. A rather long saccular prolongation of the web-membrane occurs at the tip of each 

 of the spines of the adambulacral comb. Towards the extremities of the rays the combs 

 sometimes show a tendency to separate into component spines, each being still clothed 

 with membrane. The aperture-papillae are somewhat jawbone-shaped, the thickened 

 extremity being directed adorally. 



The mouth-plates bear four or five spines on each side, which are long and stouter than 

 the spines of the adambulacral armature, both series being webbed together ; the outer 

 spine is sometimes very small and rudimentary. The secondary or superficial spines are 

 small, not so long as the innermost mouth-spines, but rather more robust. In one or two 

 plates of the specimen under notice there is a single irregular secondary spinelet, smaller 

 than and accompanying those just referred to. The first adambulacral comb after the 

 mouth-plate series has its web continued upon the aboral extremity of the mouth-plate ; 

 hence these two combs meet. 



The actino-lateral spines are of moderate length and slender ; the fringe, which extends 

 a short distance free beyond the margin of the test, is directed horizontally or in the plane 

 of the actinal surface. 



