118 THE VOYAGE OF H.M.S. CHALLENGER. 
larger than the rest, occupy the median radial line, and the other paxille form series parallel 
to this and more widely spaced ; only the median series extending along the outer fifth of 
theray. Throughout the ray, except at the extreme base, the intermediate paxillar area is 
distinctly narrower than the breadth of the supero-marginal plates. The primary embryonic 
apical plates are discernible, and are rather larger than the other plates. Several series of 
rather smaller intermediate paxille intervene between the cycle of basal plates and the 
dorso-central plate, which is rather small and inconspicuous ; the periproctal aperture lies 
external to this. The paxille decrease in size as they recede from the neighbourhood of 
the primary plates and approach the margin and extremity of the rays. The paxille 
consist of fifteen to twenty short, truncate, polygonal granules, rather well spaced, borne 
on the tabulum, and often with numerous much smaller cilia-like spinelets appearing 
at the periphery. In the series of paxillee forming the median radial line, the short sides 
of the paxillze which form the adoral and aboral extremities always have a small series 
of these cilia-like spinelets directed towards the corresponding set of the adjacent plate, 
cuarding like comb-formed pedicellariz the little spaces between adjacent plates men- 
tioned above. ‘The madreporiform body is small and slightly sunken ; it is subtriangular 
in outline, and lies external to its adjacent basal plate (paxilla), the area of which is some- 
what larger. Its position is about midway between the centre of the disk and the inner 
margin of the supero-marginal plates. 
The marginal plates form a uniformly rounded lateral wall to the disk and rays, and 
present the appearance, when seen from above, of a broad well-defined marginal border, 
which is especially emphasised in consequence of the whole intermediate paxillar area 
being sunken below the level of the supero-marginal plates. The central area of the 
disk is shghtly inflated and convex. The supero-marginal plates are thirty-one in number 
from the median interradial line to the extremity ; and are greater both in breadth and 
height than in length. The actual height of the innermost plates in the interbrachial 
angle is nearly twice the length, but the measurement along the surface of curvature,— 
z.e., the dimension at right angles to the line of length is approximately proportional to the 
length as five to two. The succeeding plates are a little longer than the foregoing, and their 
height is rather less; but they decrease as they proceed along the ray, and the relative 
proportions of length, height, and breadth also become less. All the plates are slightly 
convex along their median line perpendicular to the axis of the ray. The supero-marginal 
plates bear no spines, but are covered with rather widely spaced, large, semiglobular, semi- 
transparent granules which diminish in size near the margins of the plate, a subregular 
lineal series of small granules standing at the lateral margins. The odd terminal plate is 
small and shield-shaped ; and the tip of the ray is slightly curved upward. 
The infero-marginal plates are thirty in number, and although they approximately 
agree in length with the companion plates of the superior series the divisional sutures 
are not always directly sub-imposed. The infero-marginal plates bear no spines, but are 
