486 THE VOYAGE OE H.M.S. CHALLENGER. 



usually the middle one, or sometimes the seeoud from the outside, is nearly equal in length 

 to the breadth of the furrow, across which it stretches horizontally, whilst the two outer 

 spines radiate downward and aborally. Consequent on this position of the median spiue, 

 the spinelets which compose the outer half of the comb radiate very widely apart. The 

 connecting web is hue and semitransparent, very deeply indented between the spinelets, 

 which gives the combs a remarkable appearance ; and the web is continued from the 

 outer spine of the comb down to the base of the aperture-papilla, and not out along the 

 actino-lateral spine. The aperture-papilla, which is represented by a small conical spinelet 

 placed on the outer edge of the adambulacral plate, and between the bases of the actino- 

 lateral spines, is covered with membrane, that of the actinal area being stretched over it, 

 giving it the form of a small subtriangular or conical peak. 



The mouth-plates are of moderate length, with widely expanded lateral flanges. The 

 keel along the line of junction, which is prominent, forms a well-developed peak aborally, 

 hidden beneath the tissue of the actinal membrane, and is produced into a point adorally. 

 On each side of this, and placed on the horizontal margin of the plates, are three mouth- 

 spines, webbed together, about equal in size to the spines of the adambulacral armature, 

 but rather more robust, the innermost spine on each plate being the longest, and the 

 other two decreasing in series. Immediately behind the three mouth-spines stands a 

 small secondary or superficial mouth-spine about the same size as, or smaller than, the inner- 

 most mouth-spine, with which it is connected by a continuation of the web ; the pseudo- 

 comb being thus bent upon itself at a sharp angle, gives a very striking character to 

 the armature of the mouth-plates. The spines which form the armature of the first 

 adambulacral plates have their bases arranged in a semicircular curve, and the spines 

 radiate at equal distances apart, and are directed downward and adorally, the middle 

 spine being longest. The spines are all webbed together, and a continuation of the 

 tissue from the outermost spine is attached to the prominent posterior or aboral 

 portion of the median keel of the mouth-plates, and there coalesces with the web of the 

 corresponding comb of the neighbouring ray, thus forming an elegant compound pair of 

 fans which arch over the aboral peak of the pair of mouth-plates. 



The actino-lateral spines are rather short and robust, not extending, or only to the 

 very slightest distance, beyond the margin. They are united by a fine semitransparent 

 membrane, slightly indented between the tips, which also forms the actinal floor of the 

 test. Probably in this species there is no free extension of the lateral or marginal 

 fringe, such as occurs in shallower water forms, but unfortunately the preservation of the 

 specimens in this portion of their structure is imperfect ; for the same reason it is difficult 

 to determine with accuracy the number of actino-lateral spines, but probably about twenty 

 to twenty-two were present ; seventeen may be counted in natural position ; the sixth 

 from the mouth appears to have been the longest. 



Colour in alcohol, brownish grev. 



