142 THE VOYAGE OF HM.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 papule 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 actinal surface of the ray is flat. 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 Pl. X XVII.) 
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 vertebra. 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. 
