BEPOKT ON THE ASTEKOIDEA. 157 



The abactiual area is covered with a thin and almost transparent membrane, which 

 is indurated with a great number of spiniferous spicules or pseudo-paxillse. These 

 consist of a circular scale-like base, from the centre of which a spine-like process rises 

 vertically, and this may be divided into two, three, or four equal spinelets, the latter 

 number being the most general in the centre of the disk, and those with one spinelet near 

 the margin. The spinelets are moderately robust, obtuse, all united at the base, and 

 radiating apart very slightly, have more or less the appearance of imperfect paxillae, 

 whilst the squamous basal plate is scarcely larger than the diameter of the vertical 

 spinous process. These pseudo-paxillae are rather crowded in the immediate centre of 

 the disk, but are elsewhere moderately and uniformly well-spaced. There are no papulae. 

 The presence of an actual anal aperture is doubtful. 



The marginal plates form a perpendicular wall of small but uniform height. The 

 upper margin of the superior series and the lower margin of the inferior series are slightly 

 bent inwards and form bevelled edges. The supero-marginal plates are eight in number, 

 counting from the median interradial line, exclusive of the terminal. The innermost 

 plates are nearly twice as long as high, and the outermost are higher than long. The 

 two innermost plates on each side of the median interradial line each bear a single, very 

 minute, conical spinelet near the upper margin, and directed horizontally. The two 

 outermost supero-marginal plates extend to the median radial line, where they join 

 the corresponding plates from the other side of the ray ; the two next plates are separated 

 by a narrow strip of the abactinal membrane with pseudo-paxilloe, and the remaining 

 plates fall in the disk margin. The terminal plate is slightly tumid proximally, and 

 tapers rapidly to a fine extremity, which is continued in a robust, sharply pointed, 

 terminal spine. This spine, which is longer than the plate, is placed in the median radial 

 line of the ray, and is directed outward and slightly upward from the horizontal ; two 

 smaller spinelets less than half the size of the above are placed at a lower level, one 

 on each side of the extremity of the furrow. 



The infero-marginal plates correspond in number and breadth to their superior com- 

 panions ; their height is less than the length, and varies very slightly throughout the ray. 



Seven cribriform organs are present in each interbrachial arc ; they are very narrow, 

 and the outermost are almost imperceptible. Their structure is papilliform. (See 

 PI. XXVIII.) 



The ambulacral furrows are narrow and entirely closed in by the overarching plates 

 and spinelets. The adambulacral plates are large and subcrescentric in form, with the 

 extremities truncate and the incurved margin directed towards the furrow. Their 

 armature consists of three spines to each plate, which are moderately long, slightly 

 tapering, obtuse, and rather compressed, covered with very delicate membrane, which 

 near the base unites with that of the adjoining spines, and forms a rather broad con- 

 tinuous web, by which the spines are bound together in continuous series. The spines 



