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



posture that at first sight the foremost spine of this pair appears as if it stood alone, the 

 companion spine seeming at the same time to disturb the regularity of the succeeding 

 pair. 



The mouth-plates are narrow and united, forming a sharp-ridged keel along the median 

 line, which is prolonged exteriorly into a vitreous spine-like prolongation. Their armature 

 consists of three or four very small spines on the free margin of each plate, attached at a 

 low level, and almost hidden by the overhanging prominence of the mouth-plates anteriorly. 

 The last adambulacral plate merged in the mouth-plate bears larger spines than any of 

 the others and differently shaped, there being three greatly thickened spines, and 

 sometimes a fourth much smaller at the outer end of the series, standing in line close to 

 the elevated keel of the mouth-plate ; and a smaller one on each side of the innermost of 

 these large spines forming a line of three on the furrow margin of the plate. 



The actiual interradial areas are extensive, and reach very nearly, if not quite, to 

 the extremity of the ray. They are occupied by numerous small and regularly disposed 

 plates, bearing pseudo-paxilliform groups of granules, which become more spiniform and 

 pointed on the inner part of the interradial area, whilst those near the margin and along 

 the ray are low, subprismatic, and roundly truncate. The form of the groups on the inner 

 part of the area is circular, and there are nine or ten spinelets with one central. The 

 innermost paxilla, which occupies the angle of the area next the mouth-plates, has a 

 central pedicellarian apparatus formed of three or four delicate, straight, tapering, pointed 

 spines, rather wide apart at their bases, but meeting in a point, giving a pointed conical 

 form to the pseudo-pedicellaria. Similar but smaller pedicellarise may be present on five 

 or six of the paxillse adjacent to the adambulacral plates on the inner part of the area. 

 The groups of granules along the ray are quadrate in form and have no resemblance to 

 paxillas. The whole plate and tabulum whereon the granules or spines are borne is sunken 

 below the general actinal membrane, the groups of spines and granules alone marking the 

 separate plates ; the groups, however, are very distinctly defined, and are arranged in 

 regular lines which proceed from the adambulacral plates to the infero-marginal plates, 

 about ten being present in the lines adjacent to the median interradial line, whilst not 

 more than three or perhaps four are present midway on the ray. Within the disk the 

 adambulacral plates and their armature stand up as a prominent margin on each side of 

 the furrow, and the inner part of the interradial area has in consequence more or less of 

 an impressed character in the neighbourhood of the mouth-plates. 



The anal aperture is subcentral and distinct. 



The madreporiform body is small and slightly convex. It lies midway between the 

 centre of the disk and the marginal plates, and is bounded on its adcentral side by a large 

 crescentiform plate larger than any of the others on the abactinal area. The striations 

 are fine and comparatively straight from the point of flexure, which is usually angular. 



The ambulacral tube-feet have a fleshy sucker disk. 



