518 THE VOYAGE OF H.M.S. CHALLENGER. 



and nearly twice as long, the large sacculus usually taking a pointed or sublanceolate 

 form, whilst the small investment of the abortive aboral spine is generally rounded and 

 somewhat knobbed. The aperture-papillae are remarkably large and elongately oval, and 

 are, with their membrane, acumino-spatulate in shape, and much broader than the sacculated 

 spinelets on the adambulacral plates, and often nearly as long. 



The mouth-plates are short and comparatively broad, with widely expanded lateral 

 flanges. No prominent keel is formed along the line of junction, which is flatly rounded. 

 Two robustly clothed, rather short, obtuse secondary or superficial spines are present on 

 each plate, one near the adoral edge, the other near the middle of the plate, both maintaining 

 a wide space between their corresponding spines on the adjoining plate. About four 

 mouth-spines proper stand on the horizontal margin of each plate, which are moderately 

 long, very wide at the base, and sharply tapering, the external one sometimes being 

 reduplified. 



The actino-lateral spines are delicate, well spaced, and the longest is about the fifteenth 

 from the mouth ; none meet in the interradial line, but are widely separate even at the 

 summit of the interbrachial arc ; the spines vary but little in length up to this point, 

 but dimiuish very rapidly beyond. 



Colour in alcohol, white. 



Locality. — Station 146. Between Marion Island and the Crozet Islands. December 

 29, 1873. Lat. 46° 46' 0" S., long. 45° 31' 0" E. Depth 1375 fathoms. Globigerina 

 ooze. Bottom temperature 35°'6 Fahr. ; surface temperature 43°"0 Fahr. 



Remarks. — Hymenaster graniferus is a very distinct form, characterised by the pre- 

 sence of granular bodies in the supradorsal membrane, by the simplicity of the muscular 

 fibres in the membrane, by the paucity of the spiracula, and by the armature of the 

 adambulacral plates and mouth-plates. 



17. Hymenaster geometricus, Sladen (PI. XCII. figs. 2 and 3; PL XCIII. figs. 4-6). 



Hymenaster geometricus, Sladen, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 234. 



Marginal contour substellate. Interbrachial arcs well rounded, the minor radius bein^ 

 in the proportion of 5 2 "3 per cent. E = 42 mm. ; r = 22 mm., approximately. Pays greatly 

 attenuated and tapering, with the fringe almost, if not quite, aborted towards the ex- 

 tremities. Abactinal surface uniformly flat. Actinal surface prominently convex. 



The supradorsal membrane is thin and semitransparent, supported by extremely 

 delicate thread-like fibres, which form a regular pattern upon the disk. Usually six 

 fibres proceed from the tip of each paxilla-spinelet, and pass to the tips of the neigh- 

 bouring spinelets ; as these are all equidistant, it follows that a series of regular 

 interpeuetraut hexagons is produced. The fibres are all of uniform length, and do not 

 cross over or under one another as in Hymenaster pullatus. The spaces marked out or 



