REPORT ON THE ASTEROIDEA. 521 



19. Hymenaster membranaceus, Wyville Thomson (PI. 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, 1862, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 237. 



Marginal contour subpentagonal. Interbrachial arcs wide and flat, the minor radius 

 in the proportion of 62 per cent, or less. R = 35 mm. approximately ; r = 22 mm. Pays 

 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 paxillse-spinelets, passing from 

 one to each of those in the vicinity and crossing one another in all directions, but without 

 mero-ino- or forniinp- 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. Each valve is composed of at least ten radiating spines, their 

 bases of articulation forming a promineut semicircular boss in each radius. 



The ambulacra! furrows are narrow and deep, scarcely petaloid, although much 

 narrower near the mouth and rapidly tapering at the extremity ; the. adambulacral plates 

 are hifh. 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-papillae 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. 



(zool. CHALL. EXP. — PART LI. — 1888.) 66 



