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 ares are acute; and there is a sharply 
defined steep sulcus on the disk at the summit of each are. 
The abactinal surface is beset with small paxilliform groups of spinelets. The paxillee 
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 paxille are arranged with more 
or less regularity in longitudinal lines along the rays, and single papulz occur in the 
interspaces ; occasionally two may be found together. 
Of the marginal plates the superior series are small, and scarcely distinguishable from 
the paxillee of the abactinal surface. The infero-marginal series resemble large paxille, 
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 paxille. 
These marginal paxille 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. 
