KEPORT ON TILE ASTEROIDEA. 165 



4. Hyphalaster planus, Sladen (PI. XXV. figs. 1-3 ; PI. XXVIII. figs. 9-12). 

 Hyplialaster planus, Sladen, 1883, Jourru Linn. Soc. Lond. (Zool.), vol. xvii. p. 242. 



Pays five. R = 35 mm. ; r = 15 mm. R < 2*5 r. 



Marginal contour stellato-pentagonoid. The rays are of moderate length, and com- 

 paratively slender from the disk outwards. The disk is depressed and not higher than 

 the supero-marginal plates, although apparently capable of a slight inflation. The 

 minor radius is in the proportion of 42 per cent. The interbrachial arcs are very 

 "wide and more or less flattened, which gives a strongly marked pentagonal aspect to 

 the large disk. 



The abactinal area is covered with small closely crowded paxillse, which are limited 

 to the disk proper, and extend very slightly upon the area at the base of the rays ; the 

 median abactinal area of the ray being covered with membrane beset with small 

 squamiform plates. The paxillse are small, and composed of four to six short and 

 comparatively robust spinelets, and are so closely placed as to almost give the appear- 

 ance to the disk of being coarsely granulated, when seen without a magnifying glass. 

 A prominent conical epiproctal protuberance is present in the centre of the disk. 



The marginal plates constitute the entire thickness of the animal, and form a well- 

 rounded margin to the disk. Along the rays the supero-marginal plates of the opposite 

 sides do not meet, but are separated throughout the whole extent of the ray by a 

 median radial membranous area, beset with squamae. The rays are comparatively slender 

 and well rounded, having a cylindrical appearance, and proceeding somewhat abruptly 

 from the angles of the disk. The supero-marginal plates are ten or eleven in number, 

 exclusive of the terminal ocular plate. They are rather longer than high, excepting in the 

 one or two outermost plates, where the proportions may even be very slightly reversed. 



The infero-marginal plates correspond to the superior series, the length exceeding 

 the height throughout the ray. The height of the plates of the inferior series is 

 greater in the interbrachial arc than that of the superior series, whilst along the ray it 

 is much less ; and it is less also than one-half the length of the plate. There is a gradual 

 but very striking diminution in the size of the plates of both series as they pass from 

 the disk along the ray. The marginal plates are smooth and covered with a very fine 

 membrane ; and all are devoid of spines excepting the terminal ocular plate. The 

 terminal plate is not large or conspicuous, its size being in serial proportion to the 

 neighbouring supero-marginal plates ; it forms .a blunt obtusely rounded extremity to 

 the ray, and its actinal portion is slightly curved upwards. It bears three spinelets, 

 or representatives of such appendages — one, which is short, robust, and conical, placed 

 at the extremity in the median radial Line and directed vertically ; and a pair, one placed 

 on each side, at a lower level, but quite in front of the abactinal spine. The lateral 

 spines are probably aborted, being, at least in the specimen under notice, little more 

 than tubercles. 



