144 THE VOYAGE OF H.M.S. CHALLENGER. 



per cent. The interbrachial arcs are wide, and the immediate arm-angle is more or less 

 straight. 



The abactinal area is covered with a thin membrane. Spinelets, borne on spicule- 

 like plates, are confined to bands along the interradial lines, and a few surround the 

 epiproctal tube ; the radial areas are entirely without spinelets. The spinelets are very 

 minute, little more than microscopic spicules, elongate, cylindrical, not tapering, covered 

 with a thin membrane through which the calcareous base and shaft are distinctly visible. 

 The tubular epiproctal prolongation or anal funnel is moderately well developed, broad 

 at the base, and tapers rather rapidly. 



The marginal plates are not high, and form an almost perpendicular wall which bends 

 inward very slightly ; they do not arch over on the sides of the rays, and their curvature 

 produces only a slight rounding of the margin. The abactinal area of the ray is flat, 

 and the space which intervenes between the marginal plates of each side is covered 

 with membrane. This band is nearly uniform in breadth throughout, and is equal to 

 one-third of the greatest breadth of the ray ; it extends up to the extremity, the adoral 

 margin of the terminal plate being indented for its continuation. The supero- marginal 

 plates are eight in number, counting from the median interradial line, and exclusive 

 of the terminal. Their shape is nearly quadrate, the most inward and the most outward 

 plates having the height rather greater than the length, whilst those midway present 

 the reverse proportions. Each of the supero-marginal plates normally carries a minute 

 tubercle or rudiment of an aborted spinelet, but in some instances even this is wanting. 

 The terminal plate is not at all gibbous or tubercular, and its size and outline is con- 

 formable in every way to the regular tapering of the ray. Three small delicate spines 

 are borne on the terminal plate — one placed at the extremity, in the median line, which 

 points upward and outward, and one on each side at a lower level, which are 

 separated by the furrow and directed horizontally and radiate slightly outward from 

 the axial line of the ray. 



The infero-marginal plates correspond in number to the superior series and, excepting 

 the innermost plate, have the length greater than the height. 



Three cribriform organs are present in each interbrachial arc, the lateral ones are very 

 narrow, and all are well spaced. Their structure is lamelliform. (See PL XXVII.) 



The ambulacra! furrows are wide, straight, or very slightly petaloid, and open. Each 

 adambulacral plate bears a single spinelet, and also a rather large segmental pit and 

 papilla. The spine, which is placed at the adoral extremity of the plate, is comparatively 

 robust, cylindrical, sbghtly tapering, and longer than half the breadth of the furrow, over 

 which it is directed horizontally and slightly inwards (adorally). The segmental papilla 

 is subcircular iu shape, nearly half as large as the whole adambulacral plate, and is 

 situated midway between the bases of the spinelets on the two adjacent adambulacral 

 plates, the margin towards the furrow being straight. (See PI. XXVII. fig. 7.) Towards 



