644 THE VOYAGE OF H.M.S. CHALLENGER. 
the plate is somewhat broader than the aboral, and the margin towards the furrow is 
rather deeply concave, the greatest depth lying between the median point of the furrow 
margin and its aboral extremity, where the plate stretches prominently into the furrow, 
forming a well-defined bay along which the ambulacral tube-foot passes, and by which 
it is separated from the succeeding tube-foot. The armature of the adambulacral plates 
consists of :—(1.) a small spinelet attached to the aboral prominence of the plate above- 
mentioned, and directed horizontally over the furrow and at a slight angle in the direction 
of the ray; (2.) a longer spine standing perpendicularly and articulated on a tubercular 
elevation on the middle of the actinal surface of the plate; and (3.) a lateral spine on 
alternate plates articulated on a tubercle-like rudimentary infero-marginal plate anky- 
losed on the lateral margin of the adambulacral plate. The small inner spinelet 
is rather more than a millimetre in length, and comparatively robust at the base; it is 
often flattened and expanded at the tip, which is truncate and subspatulate, the whole 
covered with membrane bearing a rather numerous congregation of. pedicellariz. The 
perpendicular spine is delicate and tapering; the longest are about 5 mm. in length, but 
at 80 or 90 mm. from the disk they are not more than 2 to 3 mm., and their length 
generally appears to be rather irregular; they are encased in a membranous sheath, with 
a more or less elongate saccular prolongation, and the whole is covered with crowded pedi- 
cellarize. On the ovarial region the distal extremity of these spines is usually expanded 
like the proximal articulatory base, and is truncate, which gives them a robust clavate 
appearance. ‘The lateral spines are of great delicacy; the longest measure 16 to 18 mm., 
and they are encased in a sheath of very delicate membrane with crowded pedicellariz, 
and there is a comparatively large saccular knob at the extremity. The pedicellariz are 
exceedingly small and attached to the membrane by long thread-like stalks. Unfor- 
tunately, very few of these spinelets are to be found unbroken, owing to the extreme 
fragility and delicacy of the specimen, and probably also to the difficulty in detaching 
it from the hempen tangles. It is much shattered, and I am therefore unable to say with 
accuracy what the general habit of the ray would be in comparison with that of other 
species, but I am disposed to think that the lateral spines were relatively short in pro- 
portion to the great length of the rays, and that they were certainly more delicate than 
usual, 
The actinostome is large and wide, its diameter being about 12 mm., in a disk mea- 
suring 19 mm. The buccal membrane is of great delicacy, and semitransparent. The 
mouth-plates are small and inconspicuous, and present a remarkably straight margin 
towards the actinostome. Two small mouth-spines are borne on each plate, of a pecu- 
liarly curved, semicrescentic or semiscimitar form; they have the appearance of bending 
round until the outward prolongation of the distal extremity is at a right angle to the 
prolongation of the median line of the mouth-plates, and their shortness gives them the 
appearance of being turned back so as to fit almost close to the margin of the plate. They 
