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" S., long. 73° 46' 0" W. Depth 245 fathoms. Blue mud. Bottom tem- 

 perature 46° - Fahr. ; surface temperature 50° - 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 actinodateral spines, and by 

 the character of the network of the supradorsal membrane. 



4. Retaster insignis, Sladen (PL LXXVI. figs. 3 and 4 ; PL LXXVII. figs. 1 1 and 12). 

 Retaster insignis, Sladen, 1882, Journ. Linn. Soc. Lond. (ZooL), vol. xvi. p. 200. 



Marginal contour stellate, five-rayed. interbrachial arcs well rounded. Minor radial 

 proportion 44"4 per cent. B = 45 mm. ; r = 20 mm. (In another example R : r as 

 70 : 31.) Rays very slightly tapering, obtusely rounded at the extremities. Abactinal 

 surface moderately convex, rays uniformly rounded from the margin. Actinal surface flat 

 or subconcave, somewhat impressed round the actinostome. 



The supradorsal membrane is very conspicuously reticulated. The paxillae-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 and 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 markiuo 

 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 lipped 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 (i.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 is continued from the outermost 

 spine of the comb upon the adjacent actiuo-lateral spine. The small spines placed on the 

 margin detract very slightly from the general transverse aspect of the comb, their smallness 



