596 THE 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 defined. 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 
papule. ‘The plates bear small uniform spinelets, scarcely, if at all, larger than those upon 
the rays, and here and there, widely spaced, are large, triangular, forficiform pedicellariz ; 
much smaller, elongate, forficiform pedicellaria 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 papule. A number of small forficiform pedicellarie occur on the 
membrane, and great numbers of comparatively large forcipiform pedicellariz 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. 3 
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 pedicellaria, and within 
the margin of the furrow and at the base of the inner spine may be one or occasionally two 
very small forficiform pedicellarie, 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. 
