ASTEEOIDEA. OF H.M.S. ' CHALLENaER ' EXPEDITIOIST. 233 



plates. It bears three spiuelets, one in the median line above 

 the extremity of the ambiilacral furrow, and two beneath, all 

 close together. The infer'o-marginal plates correspond in number 

 with the dorsal series, but their length is very much greater in 

 relation to their height — fully three times. The four outermost 

 plates do not correspond in breadth with the companion supero- 

 marginal series. Cribriform organs three in number in each 

 angle, rather broad, bat well defined ; structure papilliform. 



Ambulacral furrows very narrow, quite closed-in by the over- 

 arching adambulacral plates and spinelets. Adambulacral plates 

 elongate and vertebra-like, the margin to the furrow being 

 deeply scooped out, the extremities prominent and thickened. 

 Ambulacral sjDines three, moderately long, sharply tapering and 

 slightly compressed : two stand near the adoral extremity o£ 

 the margin, equal in length, stretching over the furrow, and 

 radiating apart from one another ; the third spinelet is slightly 

 smaller, placed about midway on the margin of the adambulacral 

 plate, and is directed in the same direction as the aboral of the 

 two spines. The spinelets are invested with a very fine mem- 

 brane, which is continuous attb.eir bases, and the spinelets of one 

 side of the furrow interlock with those from the other. Behind 

 the furrow series, at the adoral end of the plate and standing on. 

 the prominent swelling, away from the margin of the furrow, a 

 small conical spinelet rises perpendicularly from the surface of 

 the plate; and this becomes more or less rudimentary as it 

 proceeds along the free portion of the ray. 



The mouth-plates, which are large, prominent, and not united 

 along the median suture, have a peculiar aj)pearance, resembling 

 the shape of a coulter in a marked degree. The mouth-spines 

 5 or 6 in number, placed side by side along the lateral margin of 

 the plate, are the same in length and character as the ambulacral 

 spines, and they interlock with the corresponding spines of the 

 neighbouring mouth-angle. About three small, aborted, tuber- 

 cular spinelets situated on the surface of the plate are probably 

 the representatives of secondary mouth-spines. The mouth- 

 spines are wide apart and unclosed at their aboral extremity, and 

 expose the odontophore. Actinal interbrachial areas rather large 

 and triangular, covered with squamous ventral plates, which are 

 narrow, elongate, imbricating, and regularly arranged in columns 

 on the outer half of the area, but become larger, broader, sub- 

 rotund and irregular as they approach the mouth-angle. The 



