AsiEEOIDEA OF H.M.S. ' CUALLENaiill ' EXPEDITION. 211 



13. Ambulacral sjjines short and compressed. 



1 . Muscular fibres ver}^ numerous in dorsal | 



membrane. | 



Web of actinal floor with cross muscular ypuUatus. 



fibres. I 



Longest actino-lateral spine 12th. J 



2, Muscular fibres not very numerous in ] 



dorsal membrane. ( membranaceus . 



No muscular fibres in web of acthial floor. T 

 Longest actino-lateral spine 17th. J 



D. More than three ambulacral spinelets. 



1. Ambulacral spinelets 3-4. 1 

 Aperture-papilla 5-7-rayed, comb-like, large. > . . coccinatus. 



1 pair of secondary mouth-spines. ) 



2. Ambulacral spinelets 3-5. 1 

 Aperture-papilla simple, small and dumpy. > ■ ■ ■ • prcecoquis. 



2 or 3 pairs of secondary mouth-spines. ) 



Htmenaster nobilis, Wyville Thomson. 



Hymenaster \xohS\\%,Wyv. Thorns. (187G), Journ. Linn. Soc. Zool. vol.xiii. 

 p. 73, fig. 11 ; Voy, of ' Challenger' Atlantic, vol. ii. p. 238; fig. 48. 



Marginal contour pentagonal, interradial angles very slightly 

 indented, the margin forming a continuous curve of small degree 

 from tip to tip. Eadii not produced, and tapering very slightly 

 beyond the natural angle of a pentagon. The lesser radius is in 

 the proportion of 71 per cent. ; i? = 1.38 millim., r=98 millim. 

 General form much depressed, slightly elevated in the centre of 

 the dorsal area. Hadial ' areas very conspicuously defined, the 

 paxillar spinelets being exclusively confined to those areas, 

 which consequently appear to stand at a higher level than the 

 wide interbrachial areas, which are destitute of spinelets and 

 covered with a thick fleshy membrane. Actinal surface flat, the 

 margins of the furrows being rather tumid or convex. 



Supradorsal membrane comparatively thin over the radial areas, 

 and rather parchment-like in appearance. The paxillaj usually bear 

 three spines, which are long, prominent, and arranged in triangle. 

 •Each spinelet raises the membrane into a sharp conical elevation, 

 each maintaining its individuality, the subtriangular area in the 

 centre of the paxillar crown remaining, however, more or less 

 elevated above the general level. The paxillas 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 likewise 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 



