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



at the margin, where a faint tendency to develop rudiments of two or three very 

 short radiating processes may be noticed. No calcareous union or connection exists 

 between individual paxillaj. Numerous small papulse occur in the interspaces, three to 

 five being present in the quadrangle formed by four neighbouring paxillaj. Their mem- 

 brane is very delicate, and they taper somewhat rapidly towards the tip, which is thickened 

 into a small knob. Owing to the manner in which the papula; taper, a comparatively 

 swollen appearance is given to their lower part. 



The marginal plates, which are small and subtubercular in appearance, are arranged in 

 superior and inferior series, thirty-seven to thirty-eight plates being present in each 

 between the median interradial line and the extremity of the ray. Each plate is rounded 

 or boss-like externally, and covered with a great number of small spinelets similar to 

 those of the paxillse, which gives them a prominent cushion-like appearance. The infero- 

 marginal plates are the largest, transversely suboval in form — the length increasing 

 towards the summit of the interbracbial arc — and bear not less than a hundred spinelets. 

 The supero-marginal plates are smaller, usually round, and are placed rather more 

 aborally than the companion plate of the lower series, the pairs standing consequently 

 slightly oblique. 



The actinal interradial areas are well developed, and the intermediate plates extend 

 up to the very extremity of the ray. The plates, which are oblong, are arranged in 

 regular transverse and slightly oblique lines between the adambulacral plates and the 

 marginal plates. Each series or column thus formed is isolated, being separated from the 

 neighbouring column by a narrow space ; and each plate in a column overlaps or imbri- 

 cates upon the next innermost plate. The number of the columns corresponds exactly to 

 that of the adambulacral plates, and is not in relation with that of the marginal plates. 

 The whole actinal area is overlaid by a uniform layer of membrane, by which the shape 

 of the individual intermediate plates is hidden from superficial observation. Each inter- 

 mediate plate bears a single paxilla near its free extremity, which is rather more robust 

 than those on the abactinal surface, and carries rather fewer spinelets, which are somewhat 

 longer and more widely expanded. The paxillse, like those on the abactinal area, are 

 naked and not invested with membrane. In consequence of the size and arrangement of 

 the intermediate plates, the actinal paxillse are more widely spaced than the abactinal 

 ones, and are disposed in regular lines which run from the adambulacral plates to the margin, 

 the lines or columns being marked off by straight furrows or wrinkles in the membrane. 

 As the paxillse are equidistantly spaced in each of these transverse rows, equally regular 

 and uniform longitudinal lines are also traceable along the ray. In the interbrachial arc 

 nine or ten paxillse stand in each transverse series, the same number being maintained 

 until about the outer fifth of the furrow. 



The adambulacral plates are broader than long, and appear to stand on the furrow 

 margin as the ter m inal plates of the transverse series of actinal intermediate plates ; 



