ASTEEOIDEA OF H.M.S. * CHALLENGER' EXPEDITION. 229 



tinctly hexagonal compound meshes, divided by fibres radiating 

 from their centre, would be produced — one primary mesh around 

 each spinelet, and secondary meshes v/ithin this, formed by each 

 of the radiating fibres. This disposition of reticulation, how- 

 ever, is by no means regular, as numerous supplementary meshes 

 and centres occur. The meshes are usually circular, oval, or sub- 

 triangular 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 dorsal 

 membrane and its system of reticulated fibres extends almost up 

 to the extreme edge of the lateral fringe. The tips of the paxillse- 

 spinelets produce slight, uniform, rounded elevations of the mem- 

 brane, distributed over the whole dorsal area, to which they give 

 a papillose or coarsely granulate appearance when seen by the 

 naked eye. Oscular orifice 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 membrane, with 

 reticulated fibres, into five regular, triangular fans, the margins 

 slightly overlapping, and, when shut down, completely closing the 

 oscular orifice. 



Ambulacral furrows lanceolate, scarcely, if at all, expanded in 

 the middle, and gradually tapering to the extremity. Ambulacral 

 spinelets 3, short, and placed obliquely — the two outermost stand- 

 ing almost at right angles to the furrow, the innermost, which is 

 slightly smaller, being placed rather in advance of, or aboral to, 

 these latter. The spines ai'e invested with an extensive trans- 

 parent membrane ; and frequently two, or even all the three, are 

 webbed together. "When single, the covering gives them a broadly 

 lanceolate shape. The aperture-papillse are very small and elon- 

 gately oval in form. 



Mouth-plates small, the pair forming a prominent though nar- 

 row ridge. Each plate carries three very robust, blunt, secon- 

 dary superficial spinelets placed in line along either side of the 

 keel, the anterior pair near to the adoral extremity, and the pos- 

 terior 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 se- 

 condary spinelets, of which series they seem to form the con- 

 tinuation, being directed downwards and ceutripetally. The 

 aboral pair of secondary spinelets are directed cenlrif ugally. The 



