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 paxillse-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 liue 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 ma3 T 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-papillae 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 ad oral 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-j)lates. Immediately above the anterior pair 

 and i>laced on the innermost part of the lateral flanges, are a pair of mouth-spines (i.e., one on 

 each plate), similar in every respect to the secondary spines just described, only slightly 



