ASTEEOIDEA OP U.M.S. ' CHALLENGER ' EXPEDITION". 20l 



margin. Under surface flat or «ubcoucave, somewhat impressed 

 round the actinostome. 



Supradorsal membrane very conspicuously reticulated. PaxilUc- 

 spiuelets prominent, arranged iu regular lines, joined by fibres 

 forming large uniform rhomboid meshes, which are rendered still 

 more distinct by the lines and the investment of the spinelets 

 being of a dark purple or black colour, whilst the dorsal mem- 

 brane generally is ashy white. The meshes are filled in with a 

 closely and regularly reticulated tissue, the interspaces of which 

 are small, equally spaced, and each puuctured with a minute spi- 

 raculum. The opposite angles of the rhomboid areas are usually 

 joined by fibres rather more robust than the rest, forming a right- 

 angled cross in the centre, and marking ofi" the reticulated area of 

 the mesh into four more or less easily distinguishable sections. 

 There are 80 to 100 or more spiracula in each mesh. The spine- 

 lets that stand at the angles of the meshes protrude more tban 

 the others, and appear like well-developed thornlets si?ringing 

 from the general surface. Oscular orifice small and constricted, 

 the spinelets of the pseudo-valves slightly prominent, their extre- 

 mities tipped with the same dark colour as the lines of reticula- 

 tion above mentioned. 



Ambulacral furrows narrow, straight, and sunken — their appa- 

 rent depth being further increased by the position of the pro- 

 minent fringe of the actino-lateral spines, which stands vertical 

 on either side of the furrow. Ambulacral spines 5, united 

 together by a web ; three standing on the margin of the plate 

 parallel with the furrow, the next (more adoral) placed more out- 

 wards and ^away from the furrow, and the fifth more outwards 

 still. The innermost (^. e. aboral) spine is very small, each suc- 

 ceeding one in the comb increasing in length ; all are compara- 

 tively short, delicate, and tapering. The membrane that unites 

 the spinelets is very fine, semitranspareut, and deeply festooned 

 between the spinelets, and is continued from the outermost spine 

 of the comb on to the adjacent actino-lateral spiue. The small 

 spines placed on the margin detract very slightly from the 

 general transverse aspect of the combs, their smallness rendering 

 them inconspicuous; they have, however, the peculiarity of 

 closing the space between their own and the next aboral comb at 

 the margin of the furrow. Aperture-papilla) small and sublan- 

 ceolate iu form, hidden in the general membrane, excepting their 



14i» 



