ASTEEOIDEA OP H.M.S. 'CHALLENGER' EXPEDITION. 239 



fibres, modify the normal arrangement of radiation from tip to tip, 

 whicli is, after all, the principle of the disposition of this structure 

 even in the species under notice. The meshes are filled in with 

 an almost hyaline tissue, punctured with two, three, or even more 

 small spiracula, each surrounded with a definite white ring. The 

 spinelets of the paxillse are not numerous, and are but slightly 

 protuberant, the tips being covered with, a little cap of membrane 

 which gives them rather a knobby appearance. The oscular ori- 

 fice is moderately large, its outer circumference at the base of the 

 valves being well defined by a pentagonal outline formed of thick- 

 ened or fibrous tissue. The five valves are regular and triangular, 

 with about 8 spines in each ; and the whole series are webbed 

 together and form a very slightly elevated pyramid when 

 closed. 



Ambulacral furrows wide and open, very slightly petaloid 

 opposite the commencement of the outer third, and rather rapidly 

 constricted towards the tip. Ambulacral spines 4 (but often 

 towards the extremity only 3), short, delicate, acicular, and 

 well spaced. Three stand on the margin of the plate parallel to 

 the furrow, the aboral smallest, the adoral nearly twice as long. 

 The fourth spine — equal in length to the last named, or even 

 longer — is placed close to it, but on the outer side and away from 

 the furrow. These two spines are present throughout the ray, 

 and maintain this position. The three marginal spines usually 

 stretch horizontally over the furrow ; but the fourth spine is 

 almost perpendicular, frequently radiating at an angle away 

 (outward) from the furrow. The spines are covered with an 

 investing membrane, which in the three marginal spines is ex- 

 panded towards the tip and gives them a claviform appearance, 

 the most adoral one of the three being more robust than the 

 others ; in the fourth or outward spine the investment is even 

 more developed, and the covered spine presents a somewhat more 

 lanceolate shape than those just referred to. The aperture-papillaa 

 are very singular in form, and consist of a comb of aboixt 5 to 7 

 radiating spinelets which spring from a common base, the central 

 spinelet being straight and much longer than the others, which 

 are curved, the two outer ones forming together a regular semi- 

 circular span, and the rest radiating within this curve, at gra- 

 dually lessening angles of divergence from the central spinelet. 

 The investing membrane by which the papilla is covered owes its 

 form in a great measure to this skeleton. It is ovate or oblate 



