482 THE VOYAGE OF H.M.S. CHALLENGER. 
Colour in alcohol, yellowish or greyish white. 
Locality.—Station 311. Off the entrance to Smyth Channel. January 11, 1876. 
Lat. 52° 45’ 30” 8, long. 73° 46’ 0” W. Depth 245 fathoms. Blue mud. Bottom tem- 
perature 46°°0 Fahr. ; surface temperature 50°°0 Fahr. 
Remarks.—Retaster gibber is distinguished from all other species of the genus by the 
rounded margin, by the shortness and peculiar posture of the actino-lateral spines, and by 
the ebaracter of the network of the supradorsal mem|rane. 
4, Retaster insignis, Sladen (Pl. LXXVI. figs. 3 and 4; Pl. LXXVII. figs. 11 and 12). 
Retaster insignis, Sladen, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 200. 
Marginal contour stellate, five-rayed. Interbrachial ares well rounded. Minor radial 
proportion 44:4 per cent. R= 45 mm.; 7 = 20 mm. (In another example R: 7 as 
70:81.) Rays very slightly tapering, obtusely rounded at the extremities. Abactinal 
surface moderately convex, rays uniformly rounded from the margin. Actinal surface flat 
or subeconcave, somewhat impressed round the actinostome. 
The supradorsal membrane is very conspicuously reticulated. The paxillee-spinelets 
are prominent, arranged in regular lines, joined by fibres forming large uniform rhomboidal 
meshes, which are rendered still more distinct by the lines and the investment of the 
spinelets being of a dark purple or black colour, whilst the supradorsal membrane generally 
is ashy white. ‘The meshes are filled in with a closely aud regularly reticulate tissue, the 
interspaces of which are small, equally spaced, and each punctured with a minute 
spiraculum. The opposite angles of the rhomboidal areas are usually joined by fibres 
rather more robust than the rest, forming a right-angled cross in the centre, and marking 
off the reticulated area of the mesh into four more or less easily distinguishable sections. 
There are eighty to one hundred or more spiracula in each mesh. The spinelets which 
stand at the angles of the meshes protrude more than the others, and appear like well- 
developed thornlets springing from the general surface. The oscular orifice is small and 
constricted, the spinelets of the pseudo-valves are slightly prominent, their extremities 
being tipped with the same dark colour as the lines of reticulation above mentioned. 
The ambulacral furrows are narrow, straight, and sunken, their apparent depth being 
further increased by the position of the prominent fringe of the actino-lateral spines, which 
stand vertically on each side of the furrow. The armature of the adambulacral plates consists 
of five spines, united together by a web, three standing on the margin of the plate parallel 
to the furrow, the next (more adoral) placed more outwards and away from the furrow, and 
the fifth more outward still. The innermost (¢.e. most aboral) spine of the furrow series is 
very small, each succeeding one in the comb increasing in length; all are comparatively 
short, delicate, and tapering. The membrane which unites the spinelets is very fine, semi- 
transparent, and deeply festooned between the spinelets, and 1s continued from the outermost 
spine of the comb upon the adjacent actino-lateral spine. The small spines placed on the 
margin detract very slightly from the general transverse aspect of the comb, their smallness 
