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 bands 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 witha 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 frmge. The tips of the paxillee-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-papille 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 
