ASTEROIDEA OF H.M.S. ' CHALLENGER ' EXPEDITION. 219 



rays proceeding from a truly pentagonal body-disk, the lesser 

 radius being in the proportion of 48-50 per cent. ; i2=22 millim., 

 r=10 5 millim. (tlie largest example). When viewed from above, 

 the rays seem comparatively small and have the appearance of 

 springing somewhat rapidly from the angles of the disk, the 

 interbrachial angle being very wide and not unfrequently quite 

 straight or even curved outward, rather than rounded. Disk 

 more or less gibbous and inflated, the height being sometimes 

 equal to one third of the greatest diameter, but generally less. 

 The arching or inflation of the dorsal surface extends along the 

 ray and tapers ofi" with a graceful curve towards the extremity, 

 which causes the rays to have a very short appearance when 

 seen in profile. 



Dorsal area covered with a thick coriaceous integument, the 

 usual meshwork skeleton of calcareous plates being altogether 

 wanting. The membrane is indurated with a number of minute 

 circular spicules, some of which bear a vertical spinelet, resembling 

 the surface-spicules of Thyonidium and other Holothuroids. 

 These spinelets are sparsely distributed over the central portion 

 of the dorsal area and along bands that run therefrom to the 

 arm-angle, in the median interradial line. The spinelets are 

 long and thin, and being made up of fine calcareous rods united 

 by short transverse dissepiments present, under the microscope, 

 a very open structure somewhat resembling the delicate hair-like 

 spines of certain irregular echinoids. The spinelets are clothed 

 with thick investing membrane, which not unfrequently develops 

 a knob at the extremity, and gives a club-shaped character to the 

 appendage. A more or less prominent tubular epiproctal pro- 

 longation is present in the centre of the disk ; in some examples 

 measuring between two and three millim. in length, btit shorter 

 in others. It is a subcylindrical tube less than a millimetre in 

 diameter, springing directly from the dorsal area, tapering very 

 slightly towards the extremity, and is indurated with a close 

 plating of very minute spinulate spicules. 



Marginal plates form a deep conspicuous band, and stand as a 

 perpendicular wall in the interbrachial angle, bending gently 

 inward above and below. Along the rays the supero- marginal 

 series arch well over on the dorsal surface, and leave only a very 

 constricted space along the median line of the ray between the 

 corresponding plates of either side. The superior series are 6 

 or 7 in number exclusive of the terminal, are bounded by straight 



