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 pedicellarie 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 proniinently 
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 papule in each, the calcareous 
divisions between them being 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 paxillee 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” 6, long. 134° 4’ 30” E. Depth 800 fathoms. Green 
mud. Bottom temperature 39°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 actmal 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 paxille, by the general absence 
of pedicellarize (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. (Pl. VI. figs. 5 and 6; Pl. VIL 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 
