370 THE VOYAGE OF ILM.S. CHALLENGER. 
and are of equal size, short, compressed, lanceolate, tapering to a sharp point, and invested 
with membrane, which adds to the apparent breadth of their base. The outer spines on 
the actinal surface of the plate 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 stand- 
ing 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 on the actinal surface of the plate 
are of uniform size, eylindro-conical in shape, rather obtusely pointed, and covered with 
membrane, 
The mouth-plates form a triangular mouth-angle, not prominent or protuberant super- 
ficially, and perfectly conformable with the triangular outline of the interradial area, The 
mouth-aperture is completely closed, and the arrangement of the armature of the mouth- 
plates is suggestive of that in certain Pentagonasteridw, The mouth-spines are short, 
robust, and stand perpendicularly, 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 tapering, 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 belong- 
ing to the adambulacral 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 pre- 
sent on the mouth-plates. 
Remarks. —The form above described is nearly allied to Rhegaster tumidus, Stuxberg, 
sp. ‘The following appear to be the chief points of difference. The length of the ray is 
much less in Rhegaster murrayi, the radial proportions being for Rhegaster murrayi, 
R= 13 r, and for Rhegaster tumidus, R=1°9 rv, in specimens of the same size. ‘The rays 
are consequently much less defined, and are more widely expanded at the base. In 
Khegaster murrayi the marginal contour is distinctly festooned by the infero-marginal 
plates, and each of these bears a croup of enlarged spinelets, neither of the characters 
being present in Rhegaster tumidus. The spines of the adambulacral armature appear 
to be more numerous in Rhegaster murrayt, the armature of the mouth-plates somewhat 
different, the papula more numerously distributed on the abactinal surface, and the character 
of the spinelets, both on the abactinal and actinal areas, more simple. 
Locality. —* Triton” Expedition : 
