40 THE VOYAGE OF H.M.S. CHALLENGER. 



very short. On the actinal surface of each plate are three robust, tapering, secondary 

 mouth-spines, two placed so that a line joining them would run parallel to the median 

 suture, and this line is continued on the outer part of the plate by one or two smaller 

 spinelets. The third large spinelet is placed opposite the interspace between the two large 

 spinelets above mentioned, midway between them and the outermost of the marginal 

 mouth-spines. 



The actinal interradial areas are very small, no*- more than eight to ten intermediate 

 plates being present in each. The two innermost may bear a small central conical spinelet 

 surrounded by a few minute miliary thornlets only. There are three complex pedicellarian 

 apparatus in each area, situated in the lateral sutures which separate the two innermost 

 intermediate or ventral plates ; these organs consist of an oval cavity equally scooped 

 out of the margins of the two adjacent plates, each margin beset with about five short, 

 compressed, pointed, " dog-tooth " shaped spinelets, directed over the cavity, and fre- 

 quently turned upwards into the same. The major axis of the cavity measures about 

 1 mm. There are also structures which I take to be very minute pedicellariae present 

 on a number of the adambulacral plates, appearing to protrude through the membrane, 

 usually on the outer part of the adoral margin. 



The anal aperture is subcentral and distinct, its margin being surrounded by a close 

 circlet of small spinelets longer than the small spinulation of the paxillse. At a little 

 distance from the aperture is a circlet of the large armed paxillse, standing more or less 

 regularly in the radial and interradial lines. 



The papulee, though confined to the base of each ray, occupy a much greater area than 

 in the other members of the genus, and are probably not comprised in a specially consti- 

 tuted papularium. They are small and widely spaced, more than fifty may be counted in 

 each area, and isolated ones extend as far as the fourth marginal plate. 



The madreporiform body, which is small, circular, and convex, is situated close to the 

 marginal plates, and its surface is striated with rather fine convoluted furrows. One of 

 the large powerfully spined paxillae stands on its adcentral side. 



Colour in alcohol, a bleached ashy white. 



Locality.— Station 232. South of Yeddo (Japan). May 12, 1875. Lat. 35° 11' 0" N., 

 long. 139° 28' 0" E. Depth 345 fathoms. Green mud. Bottom temperature 41°"1 Fahr. ; 

 surface temperature 64° "2 Fahr. 



Remarks. — This is, perhaps, the handsomest species in the genus, at any rate the 

 most striking, and is at the same time remarkably well characterised. Without referring 

 to minor points of difference, it will suffice to say that the form is at once distinguished 

 from all others by the group of large conical spines on the abactinal area of the disk, and 

 by the presence of more than one large spine arranged in transverse series on the infero- 

 marginal plates. Even without these striking features, Pontaster oxyacanthus would be 

 well marked. 



