514 THE VOYAGE OF H.M.S. CHALLENGER. 
join close up in the median interradial line with the corresponding spine of the neigh- 
bouring ray, whilst the succeeding spines diminish by regular steps. ‘The spines are 
tapering and pointed. The membrane is made up of very fine fibres. The margin is very 
slightly festooned between the tips of the spines. 
Colour in alcohol, white ; the ambulacral furrows and the tube-feet being yellowish 
brown. 
Locality.—Station 335. North of the Island of Tristan da Cunha. March 16, 1876. 
Lat. 32° 24’ 0” S., long. 13° 5’ 0” W. Depth 1425 fathoms. Pteropod ooze. Bottom 
temperature 37°°0 Fahr. ; surface temperature 73°°5 Fahr. 
Remarks.—This species differs from all other members of the genus by the remarkable 
character of the armature of the adambulacral plates and of the mouth-plates. The super- 
ficial appearance of the abactinal area is also exceedingly ornate and characteristic. 
14. Hymenaster latebrosus, Sladen (Pl. XCII. figs. 4 and 5; Pl. XCIII. figs. 7-9). 
Hymenaster latebrosus, Sladen, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 230. 
Marginal contour substellate. Interbrachial arcs sharply indented, the minor radius 
being in the proportion of 63°6 per cent. R=22 mm.; r=14 mm. _Interbrachial ares 
acute. Rays broad and subtriangular, with their margins gracefully curved outward. 
General form depressed ; abactinal surface over the rays more or less bombous ; radial areas 
not specially defined. Supradorsal membrane continuous up to the margin ; lateral fringe 
narrow, regular, and sharply indented. 
The supradorsal membrane is fine and semitransparent. The paxillee-spinelets are 
uniformly distributed over the entire area, but present no definite order of arrangement. 
The paxillee are composed of few spinelets, four or five being the general number. The 
muscular fibres are numerous and closely, though rather coarsely and irregularly, reticu- 
lated (intercrossed). The interspaces are filled in with a delicate semitransparent mem- 
brane, punctured with spiracula, usually one to a mesh, and consequently rather widely 
spaced. The oscular orifice is comparatively small, the valves lying almost level with the 
surface of the supradorsal membrane. 
The ambulacral furrows are moderately broad, and subpetaloid in outline, tapering 
gradually to the extremity along the outer third of the ray, and slightly constricted 
towards the actinostome. The armature of the adambulacral plates consists of three short, 
cylindrical spines rapidly tapering to a fine point, and covered with thin membrane. 
Each series is placed high in the furrow, and very oblique to the median line of the ray ; 
the aboral spinelet is much smaller than the other two, of which the adoral is slightly the 
longest. The aperture-papillee are small and oval or subcircular in form, and are some- 
times expanded laterally to such an extent that the breadth is greater than the length. 
The mouth-plates are comparatively small and short ; the keel is prominent, having a 
