160 THE VOYAGE OF H.M.S. CHALLENGER. 
about ten spinelets each, and the smaller groups of about five or six paxille. Outside this 
conspicuous ring of the disk there are a few large paxilla placed here and there amongst 
the general small or pseudo-paxille of the disk. 
The marginal plates, instead of forming perpendicular rounded sides, are inclined 
inwards, towards the centre, which gives a bevelled edge to the disk, and an arched 
rather than a rounded character to the upper surface of the rays. The supero-marginal 
plates do not meet in the median line of the ray, but leave a rather wide suture along 
the whole length, which expands on approaching the disk. All the marginal plates are 
longer than high, excepting perhaps the penultimate superior. The supero-marginal 
plates are ten in number exclusive of the terminal, and vary in depth very slightly from 
the median interradial line to the extremity of the ray. 
The infero-marginal plates correspond in number and breadth to the superior series, but 
diminish gradually in height as they proceed along the ray. The surface of the plates is 
perfectly smooth, and forms an even contour-line to the ray, the sutures being scarcely 
discernible except with a magnifying glass. None of the supero-marginal plates bear 
spines except the terminal. This plate is comparatively small and inconspicuous, sub- 
triangular in contour, and upturned at asharp angle to the plane of the ray, a position that 
gives a very marked character. It bears three rather short robust spines—one, which is 
somewhat the stoutest, is placed in the median radial line and directed vertically upwards ; 
the other two stand at the anterior actinal angles of the plate, and are directed outward 
and at an angle of about 45° to the single spine; in consequence of the thinning off of 
the terminal plate, the bases of these lateral spines are not far removed from that of the 
abactinal spine ; a deep indentation or sinus occurs between them, in which the ambulacral 
furrow terminates. 
Five cribriform organs are present in each interbrachial arc ; they are rather wide, and 
leave only a small band of the plate between adjacent organs, and each has a depression 
down the median line. Their structure is papilliform. (See Pl. XXVIII. fig. 20.) 
The ambulacral furrows are deep and contracted, the adambulacral plates arching 
considerably over, and their armature covering in the area when disposed for that 
purpose. The adambulacral plates are elongate and subcrescentiform, and each forms 
an angular prominence on the sides of the furrow, the angles separating to a certain 
extent the tube-feet of neighbouring segments. The armature of the adambulacral plates 
consists of four short, thin, and compressed spines on each plate, uniform in breadth 
throughout, and rounded at the extremity, which are arranged in a straight or sometimes 
slightly curved line, and at a very slight angle to the furrow, the direction of the line 
being outward from the furrow. A secondary row of five or six small granules stands 
on the outer margin of the adambulacral plates behind the furrow series, placed in a 
shghtly curved line, and these become more or less indistinct along the outer portion 
of the ray. 
