ASTEROIDEA 267 



Adambulacral plates block-like, not compressed, except on disk. Each plate carries a 

 transverse series of 3 slender compressed spines, that in furrow the least robust, and 

 most strongly curved; second spine obviously the most robust and slightly longer 

 than the third or outer spine. One or 2 spinelets usually stand adoral to outermost 



spine. 



Mouth-plates very narrov^r and convex, so that they arch high above the level of disk 

 adambulacrals. The true furrow margin is a short ridge deep in the furrow at inner end 

 of plate. It carries 2 bivalved slender pedicellariae and a slender spinelet which are 

 close to and may touch the broad junction of radial and ring nerve bands. Along the 

 superficial carina of each plate, bordering suture, are 5 or 6 prominent slender tapering 

 blunt spines, the inner almost as long as the plates. They decrease in size from inner to 

 outer which stands one-third plate length from outer end. The outer third of plate 

 carries 5 or 6 superficial spinelets, 5 or 6 along first adambulacral suture margin, and a 

 variously located bivalved pedicellaria. 



Papulae (the usual compound sort) are absent from the median area of distal third of 

 ray but appear everywhere else on the abactinal surface. They form three regular 

 longiseries on either side of abactinal area. In a general way their size is in direct 

 relation to that of surrounding paxillae. 



Madreporite concealed in smallest specimen but uncovered in the type, broadly 

 elliptical, 2-5 mm. longest diameter, with fine striae radiating from centre. 



Colour in alcohol : abactinal surface light brown ; dark brown between marginal plates, 

 which are whitish; bases of marginal spines brown. 



Young. From St. 279 are 2 specimens having R 26 mm. and R 55 mm. (both with 

 ID rays). In the smaller the single central spinelet of each ornate paxilla is quite 

 definitely longer and stouter than the peripheral— long enough to be called " enlarged" 

 — especially on the larger lateral and superomarginal paxillae. The spinelets are very 

 delicate and sharp. This disparity in size is less evident in the second, larger, specimen 

 and is lost in the adult (except on superomarginals). The ancestral form may have had 

 well-developed central paxillar spines. Pedicellariae are very rare on the abactinal 

 surface, absent from the mouth-plates, and are present on only the proximal actinals. 

 In place of the two oral pedicellariae at entrance to ambulacral furrow are two co- 

 ordinated spinelets which will develop into the valves of a pedicellaria as, in one 

 instance, the diflFerentiation is under way. 



The inferomarginal armature lacks the lowest spine of the triplacanthid plates. There 

 are thus 2 spines to a plate on proximal two-thirds of ray but with the differentiation 

 shown on the adult — the dorsal spine of alternate plates is close to the upper margin. 

 Distally the lower spine of the diplacanthid plates of the adult is lost so that the major 

 spines are 2, i, 2, i. This is true also of the second specimen. 



In the second specimen the first half dozen superomarginals may carry a pedicellaria, 

 the actinal pedicellariae extend to about the middle of ray, and a large oral pedicellaria is 

 present at mouth of furrow, definitely a development of the 2 spinelets of the small 

 specimen. A suboral pedicellaria is present on some but not all plates and there are 



