596 TILE VOYAGE OF H.M.S. CHALLENGER. 



end of the inner third or fourth, beyond which it gradually tapers to an attenuate 

 extremity. The interbrachial arcs are a mere close cleft, in consequence of the crowding 

 of the rays at the base. 



The disk is large and circular, subplane, and capable of slight inflation, slightly 

 elevated above the base of the rays, and more or less distinctly denned. The abactinal 

 surface is beset with small imbricating plates forming a wide meshed network, the 

 whole being covered with a membrane which is punctured in the interspaces by numerous 

 papula?. The plates bear small uniform spinelets, scarcely, if at all, larger than tbose upon 

 the rays, and here and there, widely spaced, are large, triangular, forficiform pedicellarise ; 

 much smaller, elongate, forficiform pedicellarise are more numerous. 



The abactinal surface of the ray on the basal portion is similarly beset with small 

 imbricating plates, which form a wide meshed network, the meshes being more or less 

 quadrangular. The longitudinally directed series of plates are, however, confined strictly 

 to the abactinal surface, and do not occur on the lateral walls of the ray, where widely 

 spaced, transverse series only are found, one opposite about every third adambulacral plate. 

 The lowest plate, which abuts on the adambulacral plates, is probably the representative 

 of an infero-marginal plate, and bears a single small, sharply pointed spinelet. The abac- 

 tinal plates do not extend far beyond the ovarial region of the ray, and the transverse bands 

 are then probably represented only by an aborted rudiment of the infero-marginal plate, 

 bearing, however, a fully developed spine, which may extend for two-thirds of the length 

 of the ray or more. The abactinal plates bear small, isolated, sharply pointed spinelets, 

 similar to those upon the disk, and the membrane which covers the interspaces is punc- 

 tured by numerous papulse. A number of small forficiform pedicellariae occur on the 

 membrane, and great numbers of comparatively large forcipiform pedicellarise are present, 

 borne on a roll or sacculus of membrane, and disposed as a thick transverse ruff or annulus, 

 isolated and well spaced from its neighbours, encircling the ray, and extending on each 

 side to the adambulacral plates. 



The ambulacral furrow occupies the greater part of the actinal surface of the ray, 

 measuring about 4 mm. at a part where the whole ray is 6'25 mm. The adambulacral 

 plates are short and narrow, inclined at a considerable angle aborally, and are separated by 

 a space nearly equal to their length occupied by muscular ligament. Their armature 

 consists of two short, cylindrical, tapering, sharply pointed spines, which diverge slightly, 

 one towards the furrow, the other outwardly. The bases of the two spinelets together 

 occupy the whole of the actinal surface of the small adambulacral plates. On the outer side 

 of the outer spine is a rather elongate tuft of small forcipiform pedicellarise, and within 

 the margin of the furrow and at the base of the inner spine may be one or occasionally two 

 very small forficiform pedicellarise. The ambulacral tube-feet, which are robust and 

 crowded, are biserial in their arrangement, and have a small, button-like, centrally invagi- 

 nated terminal disk. 



