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



the extremity, which is obtusely pointed. The abactinal surface of the disk is slightly 

 inflated, that of the rays arched, with a tendency towards a carinate appearance. The 

 actinal surface is subplane. The interbrachial arcs are acute ; and there is a sharply 

 defined steep sulcus on the disk at the summit of each arc. 



The abactinal surface is beset with small paxilliform groups of spinelets. The paxillas 

 are rather widely spaced, and the medium-sized ones near the base of the ray are com- 

 posed of five or six very short spinelets, which are denticulate at the extremity but not 

 tapering, and usually radiate only slightly apart. The paxillse are arranged with more 

 or less regularity in longitudinal lines along the rays, and single papula? occur in the 

 interspaces ; occasionally two may be found together. 



Of the marginal plates the superior series are small, and scarcely distinguishable from 

 the paxilke of the abactinal surface. The infero-marginal series resemble large paxillae, 

 with a broad and massive compressed pedicle, having the major axis placed at right 

 angles to the median line of the ray, and surmounted by a crown of about ten to twelve 

 spinelets, which are larger and more robust than the spinelets of the abactinal paxillae. 

 These marginal paxillre are tolerably well spaced, and there are about twenty-six between 

 the median interradial line and the extremity ; those on the outer part of the ray 

 becoming smaller in size, and the pedicle being reduced to a subtubercular eminence. 



The adambulacral plates are short but broad, and their armature consists of two series 

 of spines. (1.) A furrow series of short, skin-covered, slightly tapering spinelets, which 

 are four in number near the mouth, and then three up to the middle of the ray ; beyond 

 this only two are present, and probably only one at the extremity. When four spines 

 are present, the adoral and aboral spines of the series are generally smaller than the 

 others. (2.) On the actinal surface of the plate is a transverse lineal series of four or 

 five large, robust, cylindrical, obtusely-rounded and thickly skin-covered spinelets, which 

 diminish in size as they proceed outward, and are reduced to three in number at the 

 extremity of the ray. 



The mouth-plates are large and elongate. Their armature consists of a marginal series 

 of eight spinelets on each plate. The innermost three are larger than the others, which 

 decrease slightly as they recede from the mouth. All are skin-covered, but there is no 

 development of a definite membranous web. On the actinal surface of each plate is a 

 lineal series of three or four skin-covered spinelets, which diminish in size as they recede 

 from the mouth ; the foremost three are long, robust at the base and tapering, but the 

 fourth or outermost is usually very small. 



The actinal interradial areas of the disk, which are narrow, are occupied by small inter- 

 mediate plates bearing tufts of short, rather robust spinelets, about three to five in each. 



The madreporiform body, which is very small and inconspicuous, is situated nearer the 

 centre of the disk than midway between that point and the margin ; the striations upon 

 its surface are fine, and their direction is more or less centrifugal. 



