REPORT ON THE ASTEROIDEA. 481 



may be recognised readily by the structure of the paxillae, the resultant character of the 

 abactinal surface, and the armature of the adambulacral plates. 



3. Retaster gibber, Sladen (PL LXXIV. figs. 5 and 6 ; PL LXXVIL figs. 7 and 8). 



Itrtaster gibber, Sladen, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 199. 



Marginal contour substellate, interbrachial arcs acute, not rounded. Minor radial pro- 

 portion about 50 per cent. It = 28 mm. ; r= 14 mm. Kays five, well rounded, tapering 

 continuously from the angle to the extremity, with sides rounded, tumid, and curving 

 over upon the actinal area as in Echinaster. Abactinal profile high, gibbous, rounded. 

 Ou the actinal area the mouth and surrounding portions are deeply sunkeu. 



The supradorsal membrane has reticulated fibrous bands, which mark it off into 

 square or rhomboidal meshes of great regularity. In the centre of each, three or four 

 paxillar spinelet-tips are visible ; and other spinelets radiate to the mesh fibres, one (or 

 occasionally two) spiracula being situated in the interspaces. The white fibres of the 

 meshes form a conspicuous feature, and the slightly protrusive tips of the spinelets impart 

 a grauulose appearance to the abactinal area, the whole being covered with a thin fleshy 

 membrane. The oscular orifice is small, closed by a number of subpapillate spinelets, 

 rather longer, more prominent, and more robust than those of the paxillae proper. 



The adambulacral furrows are narrow and deeply sunken. The combs of the adambu- 

 lacral armature, which are quite within the furrow and below the level of the test, are 

 composed of three to five spinelets, which are short, rather robust, and webbed together, 

 the membrane being very slightly indented, and with a slight knob over the end of each 

 spinelet. The ambulacral tube-feet are completely hidden by the over-arching combs. 



The mouth-plates are deeply sunken ; the mouth-spines are longer than the spines of 

 the adambulacral armature, three on each plate, the whole six being webbed together into 

 a continuous comb. Each plate bears one large isolated secondary or superficial spine, 

 longer than the mouth-spines, very robust, covered with a thick membrane, except at the 

 tip, which is translucent and sharply pointed. 



The segmental apertures are elongate and narrow, the papilla} being free on the aboral 

 side only. 



The actino-lateral spines are very short and robust, almost hidden within the furrow, 

 of which they appear to form the sides, and only protrude a short way beyond the level of 

 the test; they stand nearly perpendicular to the plane of the ray, and in some parts show 

 a tendency even to arch over the furrow slightly. This disposition, together with the 

 abortive character of the fringe, imparts a feature very different from that usually pre- 

 sented by this structure in the Pterasteridse. In the immediate angle, near the actinostome, 

 the actino-lateral spines are somewhat longer, and are laid over upon the rounded surface 

 of the interradial area, their web being continuous and forming a smooth, fleshy triangular 

 area leading up to the mouth-angle. 



(zool. CHALL. EXP. — PART LI. — 1888.) 61 



