610 THE VOYAGE OF H.M.S. CHALLENGER. 
small tubercular eminence situated a little on the aboral side of the middle of the actinal 
surface of the plate. It is 3 to 4 mm. in length at 70 mm. from the disk, cylindrical, 
tapering, and invested with a delicate membranous sheath crowded with minute pedi- 
cellarie. The lateral spines are articulated on a rudimentary infero-marginal plate anky- 
losed to the adambulacral plate, which looks like a prominent tubercular eminence near 
its aboral extremity. The spines are delicate, measuring 10 to 12 mm. in length at about 
70 mm. from the disk, and are encased in a delicate membranous sheath crowded with 
very minute pedicellarize, which develops a saccular distal prolongation. On the greater 
part of the ray, two unarmed adambulacral plates usually intervene between those which 
bear lateral spines, but on the outer part of the ray their occurrence on alternate plates 
is frequent. 
The actinostome is large, measuring about 10°5 mm. in diameter in a disk 17°5 mm. 
The buccal membrane is semitransparent, and has a slightly rugose appearance. The 
mouth-plates are comparatively large and occupy the whole breadth of the actinostomial 
ring. The adoral margin of the united pair is almost straight, and the lateral margins 
converge slightly as they approach the mouth, forming a sharp angle with the adoral 
margin. The median suture line is widely open, and the opposed margins of the plates 
are slightly raised. Each plate bears on its adoral margin two small mouth-spines. The 
inner pair are situated near the outer extremity of the adoral margin and are consequently 
wide apart; they are directed horizontally over the buccal membrane and are parallel to 
one another and the median interradial line. The outer spinelets are attached close to the 
base of the companion inner spinelet, and are directed horizontally but at a right angle 
to the direction of the inner spinelet, forming a bar across the ambulacral furrow 
at the margin of the actinostome. These marginal mouth-spines are about 1 mm. in 
length, are covered with delicate membrane, and do not appear to bear pedicellariz 
generally, though a few isolated ones may be seen on the shafts in some cases. On the 
middle of the actinal surface of the plate is a cylindro-conical secondary mouth-spine, 
thick at the base and tapering to a point, about 2°75 mm. in length, covered with delicate 
membrane crowded with very small pedicellarize, and articulated on a small tubercle. 
The madreporiform body is prominent and subtubercular, oval in outline and with 
the greater diameter in the median interradial line; the central portion of its surface 
bears a few short striation furrows, presenting a rather compact disposition in comparison 
with the structure seen in some species of Brisinga. The position of the madreporiform 
body is internal or adcentral in relation to the “ odontophore,” from which it is clearly 
distinct; the so-called odontophores are superficially visible in the lateral wall of the 
disk as elongate casque-shaped plates. 
Colour in alcohol, a bleached ashy white, with a slight pinkish or flesh-coloured shade 
on the ovarial regions of the rays and over parts of the abactinal surface of the disk, owing 
to the transparency of the membrane allowing the internal organs to be distinctly visible. 
