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" K, long. 140° 32' 0" E. Depth 1875 fathoms. Blue mud. Bottom temperature 

 35° - 3 Fahr. ; surface temperature 73 o, Fahr. 



Remarks. — Uyphalaster 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 paxillse, and by the narrow cribriform organs. 



