1"8 THE VOYAGE OF H.M.S. CHALLENGER. 



ing. The largest paxillae occur midway between the centre of the disk and the margin, 

 and at the base of the rajs. These consist of a comparatively large, convex tabulum, 

 covered with rather coarse hemispherical granules, having more or less of a mulberry 

 form, with a fringe-like series at the margin of the tabulum of short, equal, skin-covered 

 papilliform spinelets, all directed horizontally. On the large paxillae there may be from 

 ten to twenty granules on the central area of the tabulum, and from sixteen to twenty in 

 the marginal series. In the medium-sized paxilla3, beyond the base of the ray and in the 

 neighbourhood of the margins throughout, there are not more than three or four of the 

 central granules, and eight to ten of the marginal series, whilst in the smaller paxillae on 

 the outer half of the ray there is seldom more than one central granule, and seven or 

 eight appear to be the normal number of marginal spinelets ; in these the membranous 

 investment is even more apparent than on the larger paxillae, and is continuous or united 

 for a considerable distance between adjacent spinelets. Upon the rays the paxillae are 

 arranged in remarkably regular, straight, transverse series, which extend uninterruptedly 

 from one series of marginal plates to that on the other side of the ray, traversing the 

 whole paxillar area of the ray. Although there is no formation of a definite median line, 

 the paxillae in the middle of the area on the inner third of the ray are distinctly larger 

 than the others in the same transverse series. 



The marginal plates are large and remarkably massive, forming a broad border to the 

 abactinal and actinal areas, and are well rounded in the lateral wall. The supero- 

 marginal plates, thirty or thirty-one in number from the median interradial Hue to the 

 extremity, have the breadth equal to about twice the length, the proportion diminishing 

 slightly towards the extremity. Their height at the summit of the interbrachial arc is 

 about twice the length, about midway on the ray it is nearly one and a half, and at the 

 extremity subequal. The width of the paxillar area is equal to that of the super- 

 marginal plate at the fifteenth plate from the median interradial line ; midway on the ray 

 it is rather greater. The plates are slightly, but rather flatly, convex along their median 

 transverse line (i.e., breadth), and are separated by well-defined channels. Their sur- 

 face is covered with rather large, uniform, tolerably well-spaced, hyaline, hemispherical 

 granules, which are abnormally deciduous, and around the margin of the plate is a fringe 

 of small, uniform, papilliform spinelets, invested with a continuous web-like membrane, 

 directed horizontally in relation to the vertical plane traversing the breadth of the plate ; 

 the fringe thus covers over the furrow between the supero-marginal plates, aud is con- 

 tinous round the end of the plate abutting on the paxillar area. There are no spines on 

 the supero-marginal plates. 



The infero-marginal plates correspond exactly to the superior series, and their length 

 is the same ; their height in the lateral view is nearly twice their length at the summit of 

 the interbrachial arc, but diminishes along the ray, being subequal or even slightly less 

 when midway. Their breadth on the actinal surface is about twice the length midway 



