236 MR. W. PERCY SLADEN OX THE 



at a lower level, one on either side of the extremity of the 

 furrow. The infero-marginal plates correspond in number and 

 breadth with their suj)erior companions ; their height is less 

 than the length and varies very slightly throughout the ray. 

 Cribriform organs seven in each angle, very narrow, the outer- 

 most almost imperceptible ; structure papilliform. 



Ambulacral furrows narrow and entirely closed-in by the 

 overarching plates and spinelets. Adambulaci'al plates large and 

 subcrescentic in form, with the extremities truncate, and the in- 

 curved margin directed towards the furrow. Ambulacral spines 

 three to each plate, moderately long, slightly tapering, obtuse, 

 and rather compressed, covered with very delicate membrane, 

 \\hich near the base unites with that of the adjoining spine?, 

 and forms a rather broad continuous web, by which the spines 

 are boiuid together in continuous series. The spines are con- 

 fined to the adoral or inner two thirds of the plate. Three to 

 five small aborted spinelets, little more than granules, stand upon 

 the surface of the plate behind and extei'ual to the furrow-series 

 and form an aborted secondary series. The adambulacral plates 

 are separated throughout the ray from the marginal series by a 

 narrow strip of membrane with scale- like plates continued from 

 the interbrachial area. 



Mouth-plates large, prominent along the line of suture, 

 forming a broad well-elevated keel in which nearly the whole of 

 the two plates is involved. The aboral extremity slopes 

 gradually, the surface of the interbrachial area being inclined 

 upward to meet it ; and adorally they likewise slope gradually. 

 Sutural junction imperfect, widely expanded aborally, exposing 

 the odoutophore. Mouth-spines proper six in each plate, i. e. 12 

 for the whole mouth-angle, the innermost one much larger and 

 more robust than the others ; there being thus two large spine- 

 lets at the innermost point of each mouth-angle directed over the 

 actinostome. The five smaller spinelets are uniform and equal, 

 less than the ambulacral spines, and are arranged equidistantly 

 along the lateral margin of the plate, and arch over the furrow in 

 continuation of the ambulacral spines. About three aborted 

 secondary spines are placed close to the margin of the plate which 

 falls in the median suture ; the middle one is largest, subconical, 

 and stands near the highest point of the keel ; the most adoral 

 is similar in shape but rather smaller, and is placed midway be- 

 tween this and the anterior extremity ; whilst the outermost one 



