ASTEROIDEA 63 



most we can speak of a warm-water and a cold-water variety of 

 one and the same species. 



4. Plutonaster Sladen. 



Disk large ; arms narrow, low, of medium length. Marginal 

 plates rather thick, not very high, covered with small grain-like 

 spines, among which may occur some large, conical spines. Oral 

 interradial areas large with several series of plates, the inner 

 series continuing to about the middle of the arm. Madreporite 

 covered by paxillse. No pedicellarise. 



One species is known to occur in the British seas, but three 

 or four more may be expected to occur there also, viz. 

 Plutonaster rigidus Sladen (Syn. PI. granulosus Perr.), known 

 from the Bay of Biscay to Madeira and the Azores, 1165-1900 m. ; 

 PL notatus Sladen, from off the Azores, ca. 1500-2180 m. ; 

 PL abhreviatus Sladen, from off the Azores, 1800 m., and PL 

 marginatus Sladen, from off the Cape Verdes and Ascension 

 (777-1700 m.). Descriptions and figures of these species are 

 given in the " Challenger " Asteroidea, as also a key to the 

 species of the genus Plutonaster (p. 82), to which the reader 

 may be referred.^ 



1. Plutonaster hifrons (Wyv. Thomson.) (Fig. 36.) 

 (Syn. Ar chaster hifrons Wyv. Thomson.) 



Both uj^per and lower marginal plates each with a prominent 

 conical spine ; there is thus a double fringe all round the disk 

 and arms. The marginal plates otherwise covered with very 

 minute spines. Adambulacral plates with a comb of 8-10 

 equal-sized furrow spines, outside which there is a single large, 

 conical spine. Plates of the oral interradial areas covered with 

 minute spines and a single large spine. Paxillse of the dorsal side 

 rather fine, in somewhat indistinct transverse series in the basal 

 part of rays along the sides. R = ca. 3-2-5 r. Colour in life creamy 

 yellow. Grows to a very considerable size, up to 20 cm. R. 

 Biology and development unknown. 



The presence of a large spine on both upper and lower 



^ Also Koehler (Resultats Camp. ScientiJ. . . . Monaco, Fasc. xxxiv.) 

 gives figures of these species. One more species, Plutonaster Edwardsi 

 Perrier, has been recorded from 39^ 33' N., 12° 11' W., 2590-2660 m. 

 (" Travailleur "). The description (Perrier's Echinodermes dii " Travailleur " 

 et du "Talisman'", p. 31.5) is, however, incomplete and without figures, 

 the species therefore scarcely recognisable. 



