252 MR. W. PEROT SLADEN OIT THE 



are narrower dorsally than the others, and the spiuelets they bear 

 are slightly longer and more robust. 



The infero -marginal plates are broader than high, and do not 

 extend laterally beyond the superior series. Each plate bears an 

 oblique comb of four marginal spines, their line of base forming 

 an angle of about 45° passing from the adoral side to the 

 aboral side of the plate. The adoral spine is the smallest and 

 the most outward, and the third from the margin the longest, 

 the second is intermediate in size, and the fourth nearly as long 

 as the third. A fifth and much smaller spine is situated on the 

 aboral side of the plate a little distance from the comb or mar- 

 ginal series ; and in the inner portion of the ray one or even two 

 similar isolated spines may be present on the aboral side of the 

 plate in lineal series. All these spines, as well as the marginal 

 series, are elongate, delicate, cylindrical, and taper to a fine point ; 

 and the marginal spines are very slightly bent. The whole of 

 the surface of the infero-marginal plates is compactly covered 

 with small, flat, roundly tipped squamules, uniform and closely 

 placed. 



The ambulacral spines are arranged in three distinct series, 

 three spiuelets in each. The inner series are of moderate length, 

 the middle spine being slightly longest, subcylindrical, and slightly 

 tapering, whilst the companion spiuelets are often slightly flat- 

 tened. The second series consists of three equal spiuelets, which 

 are shorter than the inner series, and are flat and expanded 

 towards the tip, which is roundly truncate. The outer series, 

 likewise of three spines, are similar to those of the second series, 

 the middle spinelet, however, being usually broader and more 

 flaring than the companions ; occasionally there may be an 

 additional small spine present in this sei'ies. The inner series 

 are directed more or less over the furrow ; the second series are 

 usually perpendicular ; and the outer series are directed outward 

 towards the margin of the ray. There is also a tendency in 

 the spiuelets to radiate apart. Consequent on this mode of 

 arrangement, the armature of the adambulacral plates has a very 

 widely expanded character on the whole. 



The ventral plates are small and very few in number, and con- 

 fined to the immediate interbrachial area. The spiuelets that 

 cover them are small, more or less subspatulate in form, and 

 radiate apart. 



The mouth-plates are of moderate size, each pair forming a 



