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 Islaud 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 ; PI. 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 636 per cent, Pi = 22 mm. ; r= 14 mm. Interbrachial arcs 

 acute. Rays broad and subtriaugular, 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 paxilla^-spinelets are 

 uniformly distributed over the entire area, but present no definite order of arrangement. 

 The paxillse 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-papillae 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 



