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 actinal interradial areas are extensive, and reach very nearly, if not quite, to 
the extremity of the ray. They are occupied by numerous small and recularly 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 pedicellarize may be present on five 
or six of the paxillz 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 
paxillee. 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. 
