ASTEEOIDEA OF H.M.S. ' CHALLEKGEE ' EXPEDITION. 243 



number, exclusive of the terminal ocular plate ; they are rather 

 longer than high, excepting in the one or two outermost plates, 

 where the proportions may even be very slightly reversed. The 

 infero-marginal plates correspond with the superior series, the 

 length exceeding the height throughout the ray. The height of 

 the plates of the inferior series is greater in the angle of the disk 

 than that of the superior series, whilst along the ray it is much 

 less, and it is less also than one half the length of the plate. 

 There is a gradual, bet very striking, diminution in the size of 

 the plates of both series as they pass from the disk along the ray. 

 The marginal plates are smooth and covered with a very fine mem- 

 brane ; all devoid of spines excepting the terminal ocular plate. 

 The terminal plate is not large or conspicuous, its size being in 

 serial proportion to the neighbouring supero-marginal plates, and 

 forms a blunt obtusely rounded extremity to the ray, whilst its 

 ventral portion is slightly curved upwards. It bears 3 spinelets 

 or representatives of such appendages — one, which is short, robust, 

 and conical, placed at the extremity in the median dorsal line 

 and directed vertically ; and a pair, one placed on either side, at 

 a lower level but quite in front of the dorsal spine. The laterals 

 are probably aborted, being, at least in the specimen under 

 notice, little more than tubercles. Cribriform organs 7 in each 

 angle, very widely expanded, covering nearly the whole of the 

 plates, adjacent organs almost touching in the neighbourhood of 

 the horizontal suture ; structure papilliform. 



The ambulacral furrows are narrow, and when contracted and 

 closed-in by the spinelets the sucker-feet are entirely hidden 

 from view. The adambulacral plates are elongate and subrhom- 

 boid in form, and present an angular prominence towards the 

 furrow, the adoral side of the angle being much shorter than 

 the aboral ; from this circumstance results a singularly elegant 

 festooned appearance when the furrow is viewed as a whole. 



Ambulacral spines four in number, except close to the mouth, 

 three being placed on the aboral facet of tlie furrow-margin of 

 the plate, and one on the short adoral facet. The three inner- 

 most plates of the furrow immediately succeeding the mouth- 

 plates have 5 or 6 spines, and the angular prominence into the 

 furrow is less pronounced. The ambulacral spines are uniform 

 in size and shape throughout the ray ; they are small, short, flat, 

 and terminated abruptly with a lanceolate point, and all are in con- 

 nexion at their base with the common investing membrane of 



