216 THE VOYAGE OF H.M.S. CHALLENGER. 
adambulacral plates is more stumpy in character, and there are usually two unequal, 
flattened, papilliform spinelets on the actinal surface of the plate, with a smaller one 
behind them opposite their interspace. 
14. Astropecten monacanthus, Sladen (Pl. XXXII. figs. 7 and 8; Pl. XXXVII. 
figs. 10-12). 
Astropecten monacanthus, Sladen, 1883, Journ. Linn. Soc. Lond. (Zool.), vol. xvii. p. 263. 
Rays five R=26 mm.; r=7 mm. R>85r. Another specimen measures, 
R=22 mm.; r=6 mm. Breadth of a ray at the base, 6°25 mm. 
Disk moderately large. Rays rather long and narrow, tapering rather rapidly on the 
outer portion to a pointed extremity. Interbrachial arcs rounded. 
The paxillar area is wide, measuring more than three times the width of the supero- 
marginal plates near the middle of a ray, and is very regular in composition. The paxille 
are large and well-spaced, and have one large granule-like spinelet in the centre of the 
tabulum, and seven to ten very short clavate spinelets on the circumference radiating out 
widely, almost horizontally. Occasionally on the disk, paxille have two to four central 
granules; but the single, large-sized, low granule is very characteristic. On the sides of 
the rays the paxillz are arranged in transverse lines, about three or four in each; the 
median radial band is scarcely defined. In the centre of the disk a conical peak is present, 
in the neighbourhood of which the paxillz become very small and crowded. In some cases 
the centre of the cone is invaginated. 
The supero-marginal plates, about twenty-three in number from the median interradial 
line to the extremity, which are small, higher than broad, and not tumid, form an even and 
well-rounded margin to the ray ; their extension on the abactinal surface is small in con- 
sequence of the narrowness of the plates. The surface of the plates is covered with large, 
flat-topped, well-spaced granules, which diminish a little in size at the margins ; and the 
suture or furrow between the neighbouring plates is almost hidden. The usual fine cilia, 
if present, are entirely concealed. No spinelets or tubercles of any kind are borne on the 
supero-marginal plates. 
The infero-marginal plates, which are much broader than high, do not extend beyond the 
superior series, and have a sharp subangular rounding towards the actinal surface. Each 
plate bears a single lateral spine, and no other spines are present, the whole surface of the 
plate being covered with moderately spaced, small, flat, subacuminate squamules very little 
longer than broad. Two of the squamules immediately behind the lateral spines are longer 
than the rest, and subspiniform. There is no tendency to develop spines on any other part 
of the plate. The lateral spines are short, delicate, subcylindrical, slightly compressed, 
tapering from the base to a very finely pointed extremity, and are very faintly curved ; 
they are directed horizontally from the ray at right angles to the axis, and are wide apart ; 
