510 THE VOYAGE OF H.M.S. CHALLENGER. 



spinelets, forming regular crowns, which elevate the membrane into rhomboid or penta- 

 gonal areas, apparently raised in relief, and closely placed, the margins of the raised areas 

 being more or less incurved, and the intervening spaces deep and furrow-like. The muscular 

 fibres are closely interwoven, the bands ill-defined, and the meshes irregular ; the spiracula 

 are small, and frequently two or more together, this structure being uniform over elevated 

 areas and interspaces alike. The tips of the paxillae-spmelets are only slightly protube- 

 rant ; the relief-areas which fall in the margin of the ray are smaller, more compressed, 

 and somewhat modified in form as compared with the others. About seven longitudinal 

 rows of elevated areas or paxi Ike-crowns may be counted at the base of a ray. 



The oscular orifice is small, the circumference at the base of the valves pentagonal, 

 9 - 25 mm. in diameter, and marked out by spinelets. There are five valves, which are 

 very regularly triangular, with their apices sharply pointed, and all webbed together, the 

 whole forming a regular pyramid when closed. The innermost two spines of each valve 

 stand somewhat apart, and the membrane is rather deeply drawn in between, producing 

 a well-defined furrow along the median line. The projecting tips of spinelets are pro- 

 minent at the sides of the valves. At the base of each valve are two large elevated areas, 

 formed by paxillae-crowns of six to eight spinelets, and thus nearly twice the size of the 

 other crowns upon the abactinal surface ; they are subpentagonal in shape, and the ten 

 form a conspicuous and well-defined circlet round the oscular pyramid. 



The ambulacral furrows are rather wide, straight, uniform in breadth, except at 

 the extremity, where they rapidly converge. The armature of the adambulacral plates 

 consists of three very short, cylindrical, slightly tapering spines, covered with membrane, 

 and placed in line oblique to the furrow. 



The aperture-papilla? are large and subcircular, with their investing membrane some- 

 what Japanese fan-shaped. The calcareous portion of the papillae is very flaring in habit, 

 sometimes appearing as if made up of a comb of radiating spinelets. 



The mouth-plates are small, short, but broad, and with widely expanded lateral flanges, 

 broadly rounded in front ; the keel along the line of junction is feebly developed, the 

 aboral extremity being only slightly prominent. Two secondary or superficial spines are 

 borne on slight tubercles, one near the adoral extremity and the other near the middle of 

 the plate. These spines are short, comparatively small, broad at the base, and taper slightly 

 at the tip, — the adoral pair being rather the smaller and not much larger than the spines 

 on the adambulacral plates. There are three mouth-spines, which are similar in size and 

 form to the adambulacral spines, arranged on the lateral margin of the plate and away 

 from the adoral peak of the keel. 



The actino-lateral spines are widely spaced, the difference in the length being com- 

 paratively small along the inner two-thirds of the ray. About twenty-seven spines are 

 present on each side of a ray, the sixth or seventh from the mouth being slightly longest. 

 None of the spines meet in the median interradial line, but are widely separate ; they 



