118 TIIE VOYAGE OF H.M.S. CHALLENGER. 



larger than the rest, occupy the median radial line, and the other paxilloe form series parallel 

 to this and more widely spaced ; only the median series extending along the outer fifth of 

 the ray. 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 paxillse 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 paxillse decrease in size as they recede from the neighbourhood of 

 the primary plates and approach the margin and extremity of the rays. The paxilla? 

 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 paxillge forming the median radial line, the short sides 

 of the paxillae 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, 

 guarding like comb-formed pedicellarise 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 slightly 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, — 

 i.e., the dimension at right angles to the line of length is ajaproxirnately 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 



