REPOKT ON THE ASTEKOIDEA. 169 



in consequence of the regular rounding of the plates, the ray assumes a perfectly 

 cylindro-conical form, tapering to the tip, and suggesting the appearance of a delicate 

 Belemnite. 



The supero-marginal plates are forty-three or forty-four in number, counting from 

 the tip of one ray to the tip of the neighbouring ray, exclusive of the terminal plates. 

 A pair of marginal plates stands in the median interradial line, instead of a sutural 

 division as usual in Porcellanaster. All the marginal plates are devoid of spines or 

 tubercles ; and in both the superior and inferior series the height is greater than the 

 length throughout the ray. 



The infero-marginal plates correspond exactly to the superior series, and their height 

 may also be said to be equal. A few very small conical granules, evidently loosely 

 attached, are present on the surface of the marginal plates, especially in those which 

 border the disk, and are grouped chiefly near the actinal margin of the infero-marginal 

 plates and the abactinal margin of the superior series. The terminal plate, which is very 

 small and inconspicuous, is in no way gibbous, and does not interfere with the general 

 outline of the conically tapering ray ; it is entirely devoid of spines. Seen in lateral 

 profile, the tip of the ray shows a faint tendency to an upward curve, produced by the 

 slightly elevated position of the terminal plate and the curving upward of the actinal area. 



Fourteen cribriform organs are present in each interbrachial arc ; they are very narrow 

 and their structure is papilliform. (See PI. XXIX. fig. 6.) 



The ambulacra! furrows are narrow, and when in a state of contraction, entirely conceal 

 the tube-feet. The adambulacral plates are longer than broad, but are quite incon- 

 spicuous, their form and even the divisional sutures being masked by the membrane 

 and spinelets with which they are covered. The armature of the adambulacral plates 

 consists of five spines on the furrow margin, and the series of these form a continuous 

 straight line throughout the ray, without curve or break of any kind. These spines 

 are short, robust, truncate at the extremity, and flat, their breadth being placed at right 

 angles to the furrow, and all are equidistantly spaced apart. Behind the furrow series 

 each adambulacral plate bears on its actinal surface two irregular series of smaller 

 and subconically shaped spinelets, about three standing at irregular distances next to the 

 inner spinelets, and about five in the outer series. Owing to their irregularity in number 

 and position and their tendency to group, these outer spinelets do not form the definite 

 continuous lineal series presented by the inner or furrow series of the adambulacral 

 armature. 



The mouth-plates are large, and the combined pairs form conspicuous tubercular 

 prominences. The median suture is imperfectly closed and expands at the aboral extremity 

 of the plates, exposing the odontophore. Along the free margin of the plate is disposed 

 a series of six or seven mouth-spines, similar to the spines forming the armature of the 

 adambulacral plates, the innermost being slightly largest. The surface of the plates bears 



(ZOOL. CHALL. EXP. PART LI. 18S7.) 22 



