162 MR W. PERCY SLADEN ON THE 



which extends far into the furrow and entirely separates neighbouring suckers, 

 whilst the intermediate plates are smooth, and appear to form the true furrow- 

 wall. Four ambulacral spines, which are moderately long, cylindrical, and 

 slightly tapering, are placed in single file at intervals along the edge of the 

 ridge, the innermost being usually the most delicate, and the outermost is 

 usually the shortest. Two to five small forficiform pedicellarice are attached by 

 membrane to the extremity of the delicate innermost spine. One or two small 

 ciliary spines may be present on the extreme outer edge of the adambulacral 

 plate, adjacent to the first row of longitudinal plates ; and two or three similar 

 small spines are present in the same position at the outer edge of the non- 

 prominent intermediate plates, but no spines whatever are present on the 

 surface of these plates within the furrow. 



The actinostome is deeply depressed, and the mouth-plates are entirely 

 within the cavity, and are not apposable. They are armed only with pointed, 

 moderately robust spines similar to the larger spines on the ridges of the 

 adambulacral plates. 



The madreporiform body is small and inconspicuous, and is placed external 

 to one of the interradial plates. 



The anal aperture is small, distinct, surrounded by a circlet of small ciliary 

 spines, and is placed at the side of the dorso-central plate, and consequently 

 slightly excentric in position. 



The ambulacral sucker-feet form four rows. They are rather small, sub- 

 conical, and terminated with a small but distinct fleshy sucker. 



Premature Phase.— The young form, measuring R= 11 mm. and r = 225 mm., 

 has a very remarkable appearance, owing to the prominence and distinctness of 

 the component plates of the skeleton. The disk is much higher than in the 

 adult. The dorso-central plate is prominent, and assumes the shape of a 

 rounded cone. The interradial and first radial plates are of nearly equal size, 

 and are very tumid or almost semi-globular in form. The plates of the median 

 dorsal line are large and distinct, occupying a large portion of the abactinal 

 surface of the ray. The so-called dorso-lateral series of plates form the margin 

 of the ray, and the intermediate plates are small. Between the "dorso-lateral" 

 series and the adambulacral plates there are not more than two fully- developed 

 longitudinal rows of plates, with a partially-developed series commencing to 

 appear between the latter and the adambulacral plates. The terminal (ocular) 

 plates are very large, somewhat resembling the shape of a serpent's head, and 

 are armed with one or two pairs of comparatively large robust spinelets, near 

 the extremity, which are directed outwards. 



The large plates of the disk and the median dorsal line have already a small 

 tubercle, but only some of these bear spinelets. All the plates have a few 

 widely spaced and very minute granules and microscopic ciliary spinelets. The 



