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



supero-marginal plates ; the minor radius is in the proportion of 30 per cent. The 

 interbrachial arcs are well rounded. 



The abactinal area is much contracted in consequence of the inward arching of the 

 marginal plates, the latter forming a broad border enclosing a regular pentagonal area 

 when seen from above. The whole abactinal area, excepting a very small space at the 

 base of the rays, is beset with simple spinelets, uniformly distributed over the surface 

 and moderately well spaced, the spinelets being very small, short, cylindrical, and 

 invested with membrane. In the interspaces between the spinelets, small papula? may, 

 with difficulty, be detected here and there ; in external appearance they resemble the 

 spinelets, but are rather larger and thicker. A broad tapering epiproctal tubular pro- 

 longation or anal funnel, about 3 mm. in length, is present near the centre of the disk. 



The marginal plates are high, arch inward, and form a sloping or bevelled edge to 

 the disk in the interbrachial arcs ; and a similar inclination is also continued along the 

 rays. The abactinal surface of the rays is consequently arched, and the supero-marginal 

 plates of the two sides almost meet in the median line, being separated only by a narrow 

 furrow. The actiual surface of the ray is fiat. The supero-marginal plates are higher 

 than long, and each bears a large robust spinelet nearly as long as the height of the 

 plate. The innermost spinelet, on each side of the median interradial line, is smaller 

 than any of the others. There are seven supero-marginal plates from the median 

 interradial line to the tip of the ray, exclusive of the terminal plate. This latter is large 

 and very prominent, compressed laterally, high, tubercular, and rounded abactinally, 

 and bears four spines — one placed in the median line above the termination of the 

 ambulacral furrow, and one on each side of the furrow at a lower level, all the three 

 being close together at the very extremity, whilst the fourth spine is placed in the 

 median line well back on the tubercular elevation of the terminal plate. 



The infero-marginal plates, which are nine in number, are longer than high, and do 

 not curve round on the actinal surface, but rise abruptly at a sharp angle to it. At the 

 extremity of the ray there are occasionally two or three small irregular supplementary 

 plates intercalated between the superior and inferior series, but they in no way interfere 

 with the form or position of the terminal plate. 



Three cribriform organs are present in each interbrachial arc, the median one being 

 the broadest ; they are well spaced, and each has a depression down the median line. 

 The structure is lamelliform. (See PI. XXVII.) 



The ambulacral furrows are wide, straight, and open. The adambulacral plates are 

 elongate in the direction of the ray, and their form simulates the appearance in outline 

 of caudal vertebras. Their armature consists of a single short curved spinelet, which is 

 articulated at the adoral extremity of the margin, and usually directed transversely across 

 the furrow, or sometimes at an angle more adorally, the curvature of the spine being 

 downwards. 



