62 ASTEROIDEA. 



PLUTONASTERIMl (see p. 20). 



PLUTONASTER. 



Plutonaster, Sladen, Chall. Rar. i. (1885) p. 610; id. Chall. Rep. Ast. 

 (1889) p. 81. 



A Plutonasterine with thick, generally unarmed superomarginals, 

 which extend on to the abactinal surface, without pedicellarioe. 



Key to the Species. 



Two rows of adambulacval spines .... LP. bifrons. 

 Five rows of adambulacral spines .... 2. P. pared. 



1. Plutonaster bifrons. 



Archaster bifrons, Wyv. Thorns. Depths of the Sea, (1873) p. 122, 



figs. 17 & 74. 

 Plutonaster bifrons, Sladen, Chall. Rep. Ast. (1889) p. 84. 



R =3-2 r to 4-8 r. 



Disk rather large ; arms distinct, tapering rather rapidly and quite 

 regularly. Ambulacral grooves rather wide, bordered by two rows 

 of spines ; of the inner, which are smaller, there are ordinarily nine 

 or ten, in the outer row there is only one, which is much larger, and 

 has near its base a number of spinelets. The intermediate plates 

 on the ventral surface are arranged in very regular rows, and are 

 covered by a number of fine spines among which one larger is 

 generally seen : these plates extend but a very short way on to the 

 arms. The inferomarginals are at first wider than long, but about 

 the middle of the arm are almost quadrate ; they carry a single 

 (very rarely 2 or 3) short spine ; the superomarginals are rather 

 large, but of much the same form ; their single, very rarely double, 

 spine is no longer, but the granulation with which they are covered 

 is coarser. The upper and lower plates (30-35) correspond exactly, 

 and the sutures which separate the neighbouring plates are uncom- 

 monly distinct. The paxilliform plates of the upper surface are 

 small, delicate, and regular ; the madreporite is not apparent, but its 

 position may be easily recognized by the less crowded arrangement 

 of the paxillse which hide it. 



Colour creamy yellow. 



Distribution. Both sides of North Atlantic, between Faeroe 

 Channel and Portugal, and off Massachusetts. 210 to 1360 fms. 



