164 THE VOYAGE OF H.M.S. CHALLENGER. 
suture being elevated so as to form a rounded elongate tubercular protuberance, the lateral 
margins being flattened out. Their armature consists of seven or eight mouth-spines on 
each plate, similar to those constituting the armature of the adambulacral plates, excepting 
the innermost one, which is much larger and stouter. Two large spines are thus con- 
spicuous at each mouth-angle, and are directed towards the centre, the series entirely 
closing the actinostome, which is remarkably small. The small mouth-spines upon the 
margin of the plate interlock with those of the adjacent mouth-angle, and form a con- 
tinuous series with the armature of the adambulacral plates. The rudiments of a 
secondary mouth-spine, represented by a thorn-like granule, occur on each plate, near 
the median suture, and at the highest portion of the keel. 
The actinal interradial areas are triangular in outline, flat, extensive, and covered with 
imbricating scales of more or less regularly symmetrical hexagonal form. These plates 
are broader than long, and arranged in regular series of single columns extending from 
the margin of the disk to the ambulacral furrow. Their breadth diminishes somewhat 
as they approach the margin, and consequently that of the columns also. The adam- 
bulacral plates join up to the infero-marginal plates along the whole length of the free 
portion of the ray, and there is consequently no extension of the interradial area along 
the ray. The imbricating plates bear a few widely spaced miliary tubercles or large 
granules upon their surface, usually four or five to the plate, upon which, however, they 
have no definite arrangement. 
Colour in alcohol, grey ; the paxillar area being a much darker shade, which shows a 
strong contrast with the greyish white of the marginal plates. 
Young Phase.—There is a small example of this species, which, though measuring 
only R=10 mm., r= 5 mm., so closely resembles in all respects the characters of the adult, 
that there is not the slightest hesitation in determining it specifically. Beyond the fact 
that the rays are shorter, the terminal plate more tubercular and broader, and that a less 
number of supero-marginal plates on the two sides of a ray meet in the median radial 
line, I can scarcely detect a feature worthy of mention as differentiating the immature 
from the adult form; excepting the changes in size, proportion, and number which affect 
plates and appendages normally. There are five supero-marginal plates between the 
median interradial line and the terminal in the small specimen. 
Locality—Station 237. Off Japan, south of Kawatsu. June 17, 1875. Lat. 34° 
37’ 0" N., long. 140° 32’ 0” E. Depth 1875 fathoms. Blue mud. Bottom temperature 
35°°3 Fahr. ; surface temperature 73°°0 Fahr. 
Remarks.—Hyphalaster inermis is distinguished from the other species with seven 
cribriform organs by its robust and rigid body-frame, by the supero-marginal plates 
meeting in the median radial line, by these being devoid of spinelets, by the fully 
developed paxille, and by the narrow cribriform organs. 
