496 THE VOYAGE OF H.M.S. CHALLENGER. 
the proportion of 71 per cent. R=138 mm.; 7=98 mm. General form much depressed, 
slightly elevated in the centre of the abactinal area. Radial areas very conspicuously 
defined, the paxilla-spinelets being exclusively confined to those areas, which consequently 
appear to stand at a higher level than the wide interradial areas, which are destitute of 
spinelets and covered with a thick fleshy membrane. Actinal surface flat, the margins 
of the ambulacral furrows being rather tumid or convex. 
The supradorsal membrane is comparatively thin over the radial areas and rather 
parchment-like in appearance. The paxillee usually bear three spines, which are long, 
prominent, and arranged in triangles. Each spinelet raises the membrane into a sharp 
conical elevation, each maintaining its individuality, the subtriangular area in the centre 
of the paxillee-crown remaining, however, more or less elevated above the general level. 
The paxille are arranged in regular longitudinal lines along the rays, three on each side 
of the median line, which is left free. The spinelets that compose the crowns are like- 
wise remarkably uniform in disposition, two standing aboral to the third, which is opposite 
to their interspace ; the base of the triangular crown-area thus formed is consequently at 
right angles to the median line of the ray, and the apex is directed adorally. In the 
outermost row, however, on each side of a ray, this arrangement is more or less modified 
in consequence of lateral compression of the paxille-crowns,—these being more compact 
and with the spinelets less regular in their disposition. The oscular orifice is large and 
very conspicuous, and the valves are composed of about a dozen moderately long, rather 
fine spinelets, all of which are webbed together; the valves expanding fan-like when 
open form a prominent vertical wall to the orifice. The spiracula are small, very 
numerous, closely and equidistantly placed, occupying the whole interspace between 
adjacent paxillar-crowns. Very remarkable elongated tracts or lines of spiracula and an 
accompanying wrinkled membrane extend from the paxilliferous radial areas out upon 
the fleshy interradial membrane, which has the appearance of being inlaid with them. 
These lines are slightly curved, nearly equal in length to the half of the radial area, 
placed regularly parallel to one another, and directed at an angle inward in relation to 
the direction of the ray. 
The ambulacral furrows are broad, almost straight, and very slightly petaloid. The 
tube-feet are large and robust, arranged in simple pairs. The armature of the adam- 
bulacral plates consists of one rather short spinelet, invested with a wide membrane which 
extends beyond the tip as a saccular prolongation twice as long as the spinelet itself. The 
aperture-papille are large, fleshy, subspatulate or oval, occupying nearly the whole space 
between the bases of the actino-lateral spines. 
The mouth-plates form a broad rounded ridge at their line of junction, prominent 
aborally. Each plate bears one short secondary or superficial spine, with a wide investing 
sacculus, placed about midway on the surface of the plate, quite behind the mouth-spines, 
the pair being very widely separated. The mouth-spines, which are three in number, are 
3 
