ASTEEOIDEA OF H.M.S. ' CHALLENGER ' EXPEDITION. 241 



from above. This plate bears three short and rather robust 

 spinelets — one at the terminal extremity of the plate, situated in 

 the median dorsal line, pointing in the direction of the prolonga- 

 tion of the ray, and diverging but little from the horizontal. 

 Below this spine, and at either side of it, at the angle formed by 

 the ventral edge of the plate and the terminal extremity, is a 

 somewhat smaller spinelet, pointing in the direction of the pro- 

 longation of the ventral margin of the plate. Cribriform organs 

 7 in number, narrow and well defined ; structure papilliform. 



Ambulacral furrows narrow and straight, almost completely 

 closed-in by the overarching adambulacral plates and spines, the 

 sucker-feet, which are arranged in simple pairs, being entirely 

 concealed from view. The adambulacral plates are about half as 

 broad as long, but diminish in size as they proceed outwards ; 

 and form along the ray triangular prominences projecting into 

 the farrow. Each plate bears 3 to 4 spines, rather short, rapidly 

 pointed, more or less compressed, invested with membrane, ar- 

 ranged in line along the furrow-margin of the plate and some- 

 times slightly oblique to the course of the furrow. The row of 

 spinelets can be raised at a right angle to the surface of the 

 plate, so as to allow the sucker-feet to be protruded. Traces of 

 an aborted secondary or external spinelet, represented by a mere 

 granule, may be detected at the adoral extremity of the adam- 

 bulacral plate, away from the furrow-sei'ies. 



Mouth-plates moderately large, the inner margins which fall 

 in the median suture being elevated so as to form a rounded 

 elongate tubercular protuberance, the lateral margins being- 

 flattened out. Mouth-spines 7 or 8 on each side, similar to the 

 ambulacral spines, excepting the innermost one, which is much 

 larger and stouter. Two large spines are thus conspicuous at 

 each mouth-angle and are directed towards the centre, the series 

 entirely closing the peristome, 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 continuous 

 series with the ambulacral spines. 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 interbrachial 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 



