624 THE VOYAGE OF H.M.S. CHALLENGER. 
The fleshy membrane covering the disk and the basal portion of the rays, as far as 
the ovarial region extends, is underlaid by a pavement of rather large calcareous plates, of 
irregular suboval, subhexagonal, or subrhomboid form, with their greatest diameter trans- 
verse to the direction of the ray, which imbricate upon one another. On the disk the 
membrane is sufficiently thick to hide the plates from superficial observation; on the 
ovarial region, however, the membrane is thinner, and the plates can readily be seen with 
a hand-maegnifier. On the disk the plates bear only very small spinelets, less than half a 
millimetre in length, and covered with membrane which develops a little clavate knob at 
the distal end. From their uniform size and equidistant spacing, the disk and the extreme 
base of the rays have a papillose appearance when seen with a hand-magnifier, or to the 
naked eye, as if covered with rather widely spaced granules. On the ovarial region, the 
majority of the plates bear a comparatively large spinelet, about 3 mm. (or 2-8) in length, 
articulated on a central tubercular eminence. The spine is delicate and tapering, but is 
encased in a membranous sheath, crowded with pedicellarize, which gives it a rather 
robust appearance. A few isolated pedicellariz are distributed over. the surface of the 
plates, and occasionally one or two minute spinelets similar to those on the disk, covered 
with simple membrane. ‘The pavement plates are usually composed of two or three layers 
of calcareous network, the meshes of which are somewhat larger in the centre of the plate, 
where they measure from 0°044 to 0'055 mm. in diameter. The isolated pedicellarize, which 
are sessile and of the characteristic Brisinga-form, are comparatively large, varying from 
0°185 to 0'210 mm. in length. The pedicellarize on the plates of the ovarial region are of 
similar form, but rather smaller, and measure, near the base of the spines, from 0°140 to 
0°155 mm., and diminish a little in size towards the distal end of the spine, where they 
measure about 0°120 mm. Outward beyond the ovarial regions the abactinal surface of the 
ray is covered with an exceedingly fine transparent membrane, and rather broad saddle- 
like saceuli, covered with pedicellariz, are situated on the sides of the median keel, usually 
a pair between each pair of ambulacral ossicles. The “saddles” are nearly as broad as 
long, and those of a pair on the opposite sides of the ray are often connected by a 
little irregular band of pedicellarize which crosses the median keel. The pedicellarize are 
rather small and measure about 0°133 mm. At wide intervals apart a small spicule- 
like plate, suggestive of those found in Holothurians, is met with occasionally in this 
membrane. 
The ambulacral furrow occupies nearly the whole of the actinal surface of the ray, as usual 
in the Brisingide, its width in this species being about 2°5 mm. at a place where the ray 
is 4mm. broad. The adambulacral plates consequently form a very narrow margin, which is 
rounded semicylindrically. The plates are rather short, and united by a broad tract of liga- 
ment, and their inner surface presents a semicircular concavity towards the furrow. Their 
armature consists of one small spinelet at the aboral extremity of the plate, its length 
rather less than half the width of the furrow, over which it is directed nearly horizontally 
