490 THE VOYAGE OF H.M.S. CHALLENGER. 
The supradorsal membrane is literally supported upon the tips of these spinelets. About 
nine longitudinal rows of paxilla-crowns are discernible at the base of a ray, The oscular 
orifice is small but very conspicuous, in consequence of the well-defined bosses upon which 
the valves are articulated, which are visible through the transparent membrane. The 
valves are very regular, and composed of thirteen or fourteen spines, the outer one or two 
on each side being almost aborted and hidden in the general membrane by which the 
whole series is webbed together, whilst the two innermost spines are longer than the 
others, which decrease gradually on each side and form a regular triangular fan, the base 
of which is ‘articulated on the elongate boss or modified pedicle above-mentioned. The 
bosses stand transversely and regularly central in the median line of each ray ; in conse- 
quence of their breadth the distance between two neighbouring bosses, which is very 
small, is bridged over by two or three spinelets, whilst a few short spinelets spring from 
the surface of the boss, and in this manner mark out a pentagonal outline round 
the orifice. The valves when closed lie flat over the opening, and do not form a 
pyramid. 
The ambulacral furrows are wide, lanceolate, and scarcely petaloid. The armature of 
the adambulacral plates consists of four spines (in a small specimen three), which are of 
moderate length, rather robust at the base and sharply tapering, arranged at an angle of 
such great obliquity that, roughly speaking, the series may be described as transverse, 
although the innermost spine, which is somewhat smaller than the others, is usually more 
aboral than the rest of the series. The spines are not individually invested with mem- 
brane, but are webbed together in the combs on at least the inner half of the ray. The 
web-tissue is extremely thin and very deeply indented between the spinelets ; indeed, it is 
only possible to make it out by means of favourable illumination. This character seems 
to be less general in some specimens than in others. 
The aperture-papille are small, short, and elongately oval, more or less squamous, but 
often developing small thornlets upon the surface. They are more or less hidden in mem- 
brane, and are free only on the aboral side. 
The mouth-plates are small, but rather elongate and prominent, as if compressed 
together laterally. The aboral extremity is prominent. The adoral peak is almost oblite- 
rated by the expansion of the lateral flanges. Each plate bears three superficial or secondary 
spines (in a small specimen two) not longer than the spines of the adambulacral armature, 
but nearly twice as robust, thick at the base and tapering to a fine extremity. They are 
somewhat bowed or geniculate at right angles to the line of junction to the mouth-plate. 
The anterior pair are situated at a little distance from the adoral extremity of the plates ; 
and the second pair, which are slightly larger and rather wider apart, stand midway between 
them and the aboral extremity of the mouth-plates. Immediately above the anterior pair 
and placed on the innermost part of the lateral flanges, are a pair of mouth-spines (7.e., one on 
each plate), similar in every respect to the secondary spines just described, only slightly 
