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-magnifier. 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 2-3) in length, 

 articulated on a central tubercular eminence. The spine is delicate and tapering, but is 

 encased in a membranous sheath, crowded with pedicellarise, which gives it a rather 

 robust aj^pearance. A few isolated pedicellarise 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 pedicellarise, which 

 are sessile and of the characteristic Brisinga-iorm, are comparatively large, varying from 

 0*185 to 0*210 mm. in length. The pedicellarise 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 sacculi, covered with pedicellarise, 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 pedicellarise which crosses the median keel. The pedicellarise 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 Brisingidse, its width in this species being about 2*5 mm. at a place where the ray 

 is 4 mm. 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 



