524 THE VOYAGE OF H.M.S. CHALLENGER. 
plates, the remarkable structure of the aperture-papillee, 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 precoquis, Sladen (PI. XC. figs. 5 and 6; Pl. XCI. figs. 13-15). 
Hymenaster precoquis, Sladen, 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=10mm. ; 
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 defined externally. No lateral fringe present. Actinal surface flat. 
The supradorsal membrane is very fine and semitransparent. The muscular tissue is 
very feebly developed, no definite series of fibrous bands 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 paxille 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 paxille 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-papillee 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 
