REPORT ON THE ASTEROIDEA. 521 
19. Hymenaster membranaceus, Wyville Thomson (Pl. XCII. figs. 6 and 7; PI. 
XCIII. figs. 10-12). 
Hymenaster membranaceus, Wyville Thomson, 1877, Voy. of Challenger, Atlantic, vol. i. p. 108 
(name only). 
Hymenaster membranaceus, Sladen, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 237. 
Marginal contour subpentagonal. Interbrachial ares wide and flat, the minor radius 
in the proportion of 62 per cent. or less. R=35 mm. approximately ; r=22 mm, Rays 
very narrow and tapering on their outer portions. Abactinal area almost flat. Actinal 
area slightly convex, but deeply incurved along the median interradial lines. 
The supradorsal membrane is thin and transparent, with a great number of thin 
fibrous muscular bands extended between the tips of the paxille-spinelets, passing from 
one to each of those in the vicinity and crossing one another in all directions, but without 
merging or forming a reticulated tissue in the true sense of the word. 
The tissue is semitransparent, with a few isolated spiracula here and there. The 
tips of the spinelets produce slight prominences, but there is no massing of the tissue 
or the fibres upon their extremities, which are consequently quite sharp and little con- 
‘spicuous. The oscular orifice is very large, the outer or basal circumference measuring 
12°5 mm. in diameter. Hach valve is composed of at least ten radiating spines, their 
bases of articulation forming a prominent semicircular boss in each radius. 
The ambulacral furrows are narrow and deep, scarcely petaloid, although much 
narrower near the mouth and rapidly tapering at the extremity ; the adambulacral plates 
are high. The armature of the adambulacral plates consists of three very short, stout, 
slightly tapering and slightly compressed spines, placed high in the furrow, each series 
standing in line slightly oblique to the median line of the ray, and oblique also to the 
horizontal plane of the furrow, the innermost spine being’ highest up in the furrow 
and most aboral. The spines are not webbed. The aperture-papillee are very large, and 
covered with widely expanded membrane, which gives them a squarely oval or sub- 
quadrate shape, often with a slight peak. 
The mouth-plates are rather small, sloping upward into the mouth, their aboral extremity 
being tilted downward; a prominent keel occurs along the line of junction, and the 
internal peak is rounded and very little produced. Each mouth-plate carries two 
secondary or superficial spines: one short, dumpy, obtuse, compressed, standing on the 
surface of the plate at about one-third the distance from the adoral extremity, and 
directed somewhat outward and away from the mouth; the other, rather longer but 
much less robust, placed quite at the adoral extremity, and rather geniculated sideways ; 
in fact this spinelet might almost be ranked as a mouth-spine proper, except that it stands 
at a slightly lower level and more on the plate itself. On the horizontal margin of the 
plate, and situated on the widely expanded lateral flange, are four or five small compressed 
mouth-spines, very much smaller than those just described, and similar to the spines on 
the adambulacral plates, only smaller in size. 
(z00L. CHALL. EXP.—PART LI.—1888.) 66 
