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



plates, the remarkable structure of the aperture-papillae, and the widely expanded and 

 simply armed mouth-plates in Hymenaster coccinatus, apart from its rich colouration, 

 readily distinguish the species from all other forms. 



21. Hymenaster pr&coqu is, Sladen (PI. XC. figs. 5 and 6 ; PI. XCI. figs. 13-15). 

 Hymenaster prtecoquis, SladeD, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 240. 



Marginal contour subpentagonal. Interbrachial arcs very feebly incurved, the rays 

 slightly attenuated at their extremities. Minor radial proportion 65 per cent. R = 10 mm. ; 

 r = 6'5 mm. The abactinal surface forms a uniform convex curve of low elevation, the 

 membrane arching over from margin to margin, and the radial areas being in no way 

 specially denned externally. No lateral fringe present. Actinal surface flat. 



The supradorsal membrane is very fine and semi transparent. The muscular tissue is 

 very feebly developed, no definite series of fibrous bauds being present, although under 

 considerable magnification the existence of aggregated fibres may be discerned. The 

 spiracula are comparatively large, numerous, and equally distributed, surrounded by a 

 conspicuous white ring. The paxillae are large, robust, closely placed, and usually sur- 

 mounted with five (sometimes six or seven) spinelets, which are thick and widely expanded 

 from the pedicle. The spinelets do not taper at their extremities, but expand somewhat, 

 and are flaring, elevating the membrane very slightly. The paxillse are clearly visible 

 through the transparent membrane, and about nine longitudinal rows may be counted 

 across the base of the ray. The oscular orifice is small, with the spines of the valves 

 long and slightly tapering. 



The ambulacral furrows are narrow and lanceolate, maintaining a nearly uniform 

 breadth till near the extremity. The armature of the adambulacral plates consists of three 

 to five rather long, delicate, and acicular spines, arranged on the plate in a semicircular 

 curve when the larger number are present — three usually being on the margin of the 

 furrow, and the two adoral ones standing successively more outward (away from the 

 furrow) on the plate. These two obliquely placed spinelets maintain their position 

 throughout the rays, and any diminution which takes place in the number towards the 

 extremity is manifest in the marginal or aborally placed members of the series. The most 

 aboral spine is rather shorter than the others, which are nearly uniform in length, and 

 each of them is invested with a very thin membrane, and no sacculus is produced. The 

 first adambulacral plates (nearest the mouth) bear only two spines, and these sometimes 

 are webbed together. 



The aperture-papillae are small and dumpy, the calcareous portion being little more 

 than twice (or at most three times) as long as broad, and very frequently this is bulged 

 out somewhat at the side. The papilla is not free as usual in this genus, but is clothed 

 with the general tissue of the actinal area, the aboral lateral margin alone being free and 



