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



the proportion of 71 per cent. R= 138 mm. ; r = 98 mm. General form much depressed, 

 slightly elevated in the centre of the abactinal area. Radial areas very conspicuously 

 defined, the paxillse-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 paxillse 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 subtriaugular area in the centre 

 of the paxillse-crown remaining, however, more or less elevated above the general level. 

 The paxillse are arranged in regular longitudinal lines along the rays, three on eaeh 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 paxillse-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-papillae 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 



