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



the interspace between these and the margin of the plate is one large robust conical spine 

 similar to those on the adambulacral plates ; there may also be two or three irregularly 

 placed small conical thornlets on the outer part. 



The actinal interradial areas, which are of small dimensions and very narrow, do not 

 extend beyond the third infero-marginal plate. They are slightly sunken, especially between 

 the mouth-plates and innermost marginal plates, which emphasises the convexity of the 

 former and contributes to the ornate character of the actinal aspect of the disk. The areas 

 are occupied by a small number of rather large intermediate plates, which bear upon their 

 surface a few small, pointed, thorn-like spinelets. There are five or six pedicellarise in each 

 area, composed of four or five modified spines similar to those in Pontaster forcipatus. 



The anal aperture is subcentral, large and distinct. 



The papularia are small, compact, and very conspicuous, their area being a prominently 

 convex oval. They are situated on the disk, with their outer extremity touching a line 

 drawn across the base of the ray, and contain about a dozen papula? in each, the calcareous 

 divisions between them beinsr narrow, and forming a net-like structure. 



The madreporiform body is small, oval, slightly convex, and situated about, or rather 

 more than, its own diameter distant from the inner edge of the marginal plates. The 

 striation-furrows with which its surface is grooved are rather coarse, and are more or less 

 irregular in their radiation. Several large paxillse stand near the margin of the madre- 

 porite. 



Colour in alcohol, an ashy white ; almost a bleached white on the actinal surface. 



Locality. — Station 191. In the Arafura Sea, north-west of the Arrou Islands. 

 September 23, 1874. Lat. 5° 41' 0" S., long. 134° 4' 30" E. Depth 800 fathoms. Green 

 mud. Bottom temperature 39 0, 5 Fahr. ; surface temperature 82°'2 Fahr. 



Remarks. — This species is remarkable for its close affinity to, and apparent mimicry of, 

 Pontaster forcipatus. It is, however, distinguished from that form by the smaller disk, by 

 the rounded character of the rays in the actinal aspect, by the different facies of the actinal 

 surface and of the marginal plates, by the presence of the well-developed secondary spine 

 on the infero-marginal plates, by the smallness and scarcity of the miliary granulation on 

 the marginal plates generally, by the different habit of the paxillge, by the general absence 

 of pedicellarise (excepting the few in the actinal interradial areas), by the greater pro- 

 minence of the semicircular margin of the adambulacral plates, by the slightly different 

 character of their armature, and also of that of the mouth-plates. 



9. Pontaster pristinus, n. sp. (PI. VI. figs. 5 and 6 ; PI. VII. figs. 7 and 8). 



There is a single small specimen, which, although in a young and immature stage of 

 growth, presents characters which indicate that it does not belong to any of the species 

 hitherto described. Under these circumstances I feel obliged to give it a new specific 



