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



about ten spinelets each, and the smaller groups of about five or six paxillse. Outside this 

 conspicuous ring of the disk there are a few large paxillae placed here and there amongst 

 the general small or pseudo-paxillse 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 ajxproaching 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 

 dimmish 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 a sharp 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 PI. 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 

 slightly curved line, and these become more or less indistinct along the outer portion 

 of the ray. 



