238 MR. W, PERCY SL/VDEN ON THE 



ones. The larger groups consist of ten to twelve Lirge paxillse 

 of about ten spinelets each ; and the smaller groups of about five 

 or six paxillaj. Outside this conspicuous ring of the disk there 

 are a few large paxillse placed here and there amongst the general 

 small or pseudo-paxillse of the disk. 



Marginal plates, instead of forming perpendicular rounded 

 sides, are inclined iuwards 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-margiual 

 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. Ml the marginal plates are longer than 

 high, excepting perhaps the penultimate superior. The superior 

 series are ten in number exclusive of the terminal, and vary in 

 depth very slightly from the arm-angle to the extremity of the 

 ray. The inferior series correspond in number and breadth with 

 the superior series, but diminish gradually in height as they pro- 

 ceed along the ray. The surface of the plates is perfectly smooth, 

 and forms an even contour-line to the ray, the sutures being hardly 

 discernible except wath magnifying-power. None of the supero- 

 marginal plates bear spines except the terminal. This plate is 

 comparatively small and inconspicuous, subtriangular in contour, 

 and upturned at a sharp angle from the plane of the ray, a posi- 

 tion that gives a very marked character. It bears three rather 

 short robust spines — one, which is somewhat the stoutest, is 

 placed in the median dorsal line and directed vertically upwards ; 

 the other two stand at the anterior ventral angles of the plate, 

 and are directed outward and at an angle of about 4<5° 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 dorsal spine ; a deep indentation or sinus occurs between 

 them, in which the anibulacral furrow terminates. Cribriform 

 organs five in each angle, rather wide, and leave only a small 

 band of the plate between adjacent organs; each with a depres- 

 sion down the median line ; structure papilliform. 



Ambulacra! furrows deep and coutracted ; the adambulacral 

 plates arching considerably over, and the ambulacral spinelets 

 covering-in the area when disposed for that purpose. The 

 adambulacral plates are elongate and subcresceutiform, and each 

 forms an angular prominence on the sides of the furrow, the 

 angles separating to a certain extent the sucker-feet of neigh- 



