208 MR. W. PEKCT SLADEN ON THE 



orifice is small but very conspicuous, in consequence of the 

 well-developed bosses upon Tvhicb the valves are articulated, 

 and which are visible through the transparent membrane. 

 The valves are very regular, and composed of 13 or 14 spines — 

 the outer one or two on either 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 either side and 

 form a regular triangular fan, the base of which is articu- 

 lated on the elongate boss or modified pedicel above mentioned. 

 The bosses stand transverse and regularly central in the median 

 line of each ray ; in consequence of their breadth the distance 

 between two neighbouring bosses is very small, and is bridged 

 over by two or three spinelets, whilst a few short spinelets 

 spring from the surface of the boss, and in this manner mart 

 out a pentagonal outline round the orifice. The valves when 

 closed lie flat over the opening, and do not form a pyramid. 



Ambulacral furrows wide, lanceolate, scarcely petaloid. Am- 

 bulacral spines 4 (in a small specimen 3), 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 its series. The spines are not individually 

 invested with membrane, but are webbed together into 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 illumi- 

 nation. 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 a small thornlet 

 from the surface. They are more or less hidden in membrane, 

 and free only on the aboral side. 



Mouth -plates small, but rather elongate and prominent, as if 

 compressed together laterally. Aboral extremity prominent. 

 Adoral peak almost obliterated by the expansion of the lateral 

 flanges. Each plate bears 3 superficial secondary spines (in a 

 small specimen 2), not longer than the ambulacral spines, but 

 nearly twice as robust, thick at the base, and tapering to a fine 

 extremity. They are somewhat bowed or geniculate at right 



