158 MR W PERCY SLADEN ON THE 



in size, rather widely spaced, and are directed outward, almost horizontally, 

 the angle at which they stand to the actinal surface being very small. 



Ambulacra! furrows narrow and almost uniform in breadth throughout. 

 Adambulacral plates broader than long, bearing from five to eight spines. 

 The ambulacral spines form a regular inner or furrow series, which arches over 

 and almost conceals the ambulacral sucker-feet, and three sub-regular outer 

 rows more or less clearly defined. The following is the arrangement of the 

 spinelets on the plates : — Of the inner or furrow series there are two on each 

 plate, which stand side by side and slightly oblique, especially towards the 

 end of the ray. These two spines are regular throughout the ray, and are of 

 equal size, short, compressed, lanceolate, tapering to a sharp point, and invested 

 in membrane, which adds to the apparent breadth of their base. The outer 

 spines are subject to a considerable amount of variation, both in number and 

 position. Three only may be present, each placed behind the other, external 

 to the furrow spines, forming a transverse series on the adambulacral plate, or 

 one, two, or even all three of these spines may be reduplified — the companion 

 spine usually standing rather oblique. These variations do not appear to be 

 dependent on position in the ray, but may occur in any part. All the outer 

 spines are of uniform size, cylindro-conical in shape, rather obtusely pointed, 

 and covered with membrane. 



Mouth-plates form a triangular mouth-angle, not prominent or protuberant 

 superficially, and perfectly conformable with the triangular outline of the inter- 

 radial area. The mouth-aperture is completely closed, and the arrangement of 

 the armature of the mouth-plate is suggestive of that in certain Goniasteridce. 

 The mouth-spines are short, robust, and stand perpendicular. One odd spine 

 is placed at the extreme angle, at the junction of the two plates of a mouth- 

 angle, and five similar spines, all closely placed, occupy the free or furrow 

 margin of the plate, decreasing in size as they recede from the mouth ; the odd 

 spine being the largest, the next three slightly smaller, and the two outer ones 

 much smaller. All the spines are cylindrical, slightly taper, and obtusely 

 rounded at the tip. Upon the surface of the plates, and on a line with the 

 two small outer mouth-spines, stand two short secondary or superficial mouth- 

 spines, one on each plate, very robust at the base, conical and pointed ; and, 

 further outward again, a second, but much smaller, spine behind each of the 

 secondary mouth-spines ; this small pair perhaps belonging to the adambu- 

 lacral plate adjacent to the mouth -plates. A single minute spinelet, situate 

 on the median or sutural line of the mouth-plates, stands midway between 

 each of the pairs of secondary mouth-spines ; and no other spines of any 

 description are present on the mouth-plates. 



Remarks. — The form above described is nearly allied to Rhegaster tumidm 

 (Stuxberg, sp.). The following appear to be the chief points of difference : — 



