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-papille 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 diminish very rapidly beyond. 
Colour in alcohol, white. 
Locality.—Station 146. Between Marion Island and the Crozet Islands. December 
29, 1873. Lat. 46° 46’ 0” &., 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 spinels, and by the armature of the 
adambulacral plates and mouth-plates. 
17. Hymenaster geometricus, Sladen (Pl. 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 being 
in the proportion of 52°3 per cent. R=42mm.; r=22mm.,approximately. Rays 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 
interpenetrant 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 
