REPORT ON THE ASTEROIDEA. 513 



meshes or interspaces are large and equidistant, and a more or less distinct grouping 

 round centres is distinguishable, where the fibres become massed together — these thickened 

 portions corresponding usually with the tips of the spinelets, upon which they form a cap. 

 Six to eight bauds usually radiate from these centres ; and if the plan just indicated were 

 regularly carried out, an arrangement of more or less distinctly hexagonal compound 

 meshes, divided by fibres radiating from their centre, would be produced — one primary 

 mesh around each spinelet, and secondary meshes within this formed by each of the 

 radiating fibres. This disposition of reticulation, however, is by no means regular, as 

 numerous supplementary meshes and centres occur. The meshes are usually circular, 

 oval, or subtriangular in outline, and are filled in with "a fine semitransparent membrane 

 punctured in the centre with a minute spiraculum, which is surrounded by an opaque 

 whitish ring. The supradorsal membrane and its system of reticulated fibres extends almost 

 up to the extreme edge of the lateral fringe. The tips of the paxillse-spines produce slight, 

 uniform, rounded elevations of the membrane, distributed over the whole abactinal area, 

 to which they give a papillose or coarsely granulated appearance when seen by the naked 

 eye. The oscular orifice is small, the circumference at the base of the valves being circular 

 and well marked out. The spines of the valves are webbed together by an investing mem- 

 brane, with reticulated fibres, into five regular, triangular fans, the margins overlapping 

 and, when shut down, completely closing the oscular orifice. 



The ambulacral furrows are lanceolate, scarcely, if at all, expanded in the middle, and 

 gradually tapering to the extremity. The armature of the adambulacral plates consists of 

 three short spinelets, placed obliquely — the two outermost standing at almost right angles 

 to the furrow, the innermost, which is slightly smaller, being placed rather in advance of, 

 or aboral to, these latter. The spines are invested with an extensive transparent mem- 

 brane, and frequently two, or even all the three, are webbed together. When single, the 

 covering gives them a broadly lanceolate shape. The aperture-papillae are very small and 

 elongately oval in form. 



The mouth-plates are small, the pair forming a prominent though narrow ridge. Each 

 plate carries three very robust, blunt, secondary superficial spinelets placed in line along each 

 side of the keel, the anterior pair near the adoral extremity, and the posterior pair near the 

 aboral extremity of the mouth-plate. They differ slightly in size, the adoral being smallest 

 and the aboral largest. On the horizontal margin of each plate are two mouth-spines, the 

 innermost pair immediately above the anterior secondary spinelets, of which series they 

 seem to form a continuation, being directed downward and centripetally. The aboral pair 

 of secondary spinelets are directed centrifugally. The second or outer mouth-spine is 

 very much smaller and placed away from the inner mouth-spine, somewhat isolated on 

 the margin of the plate, and is directed horizontally. 



The actino-lateral spines are about twenty-one on each side (of fairly large ones only 

 eighteen), the sixth from the mouth being longest ; this and all the spines preceding it 



(ZOOL. CHALL. EXP. PART LI. — 1888.) 65 



