ASTEEOIDEA. OF H.M.S. ' CHALLEXQEB, ' EXPEDITION". 231 



calated between the dorsal and ventral series, and entirely separates 

 them. This intermediate series may be represented by only a few 

 plates on the outer part of the ray, or may be continuous along 

 a considerable portion of the ray. Cribriform organs 7-9 in 

 number in each angle, very wide, and occupy nearly the whole of 

 the plates, in large examples adjacent organs being confluent in 

 the neighbourhood of the horizontal suture ; structure papilli- 

 form. 



The ambulacral farrows are wide when expanded, occupying 

 the whole of the actinal aspect of the ray ; but when contracted 

 are arched over and closed in by the ambulacral spines. The 

 adambulacral plates are elongate and subrhomboid, the margin 

 towards the furrow and the ventral surface being incurved, 

 suggesting the appearance of a caudal vertebra. Ambulacral 

 spines four in number, with aboral spine longest, along the 

 greater portion of the ray ; near the mouth five spines, short 

 and subequal. The spines are comparatively short, with a 

 wide robust base, thence taper and sharply pointed and slightly 

 compressed, and all radiate at diff"erent angles from the plate. 

 On the plates near the mouth tbe spines are much shorter, 

 subequal, rapidly pointed, and quite flat. The bases of these 

 spines are united by a more or less continuous membrane. In 

 large specimens a small granule is present, behind the ambulacral 

 spines, close to the adoral extremity of the adambulacral plate; 

 and near the mouth even two or three are sometimes found. 



Mouth-plates large and prominent, with a widely open median 

 suture, the apposed margins of the plates being bent downward 

 with a gentle curve until at right angles to the plane of the 

 actinal surface. There is a comb of 7 or 8 short, flat spinelets, 

 similar to those on the innermost adambulacral plates, on the 

 margin of the plate wliich abuts onto the furrow, and one or 

 two more prominent and conical ones at the innermost ex- 

 tremity, directed towards the centre of the peristome. No 

 spinelets or tubercles are present on the surface of the mouth- 

 plates, although in the largest specimen four or five granules 

 form a line parallel with the sutural margin. The actinal 

 interbrachial areas are large and elongate in the prolonga- 

 tion of the median interbrachial line, and are covered with squa- 

 mous plates, the whole being covered with a thin membrane. 

 The plates are broader than long, imbricate slightly, and are 

 arranged more or less in columns, which latter, however, may 



