62 ASTEROIDEA. 
PLUTONASTERIN & (see p. 20). 
PLUTONASTER. 
Plutonaster, Sladen, Chall. Nar. 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 pedicellariz. 
Key to the Species. 
Two rows of adambulacral spines .... 1. P. bifrons. 
Five rows of adambulacral spines.... 2. P. pareli. 
1. Plutonaster bifrons. 
Archaster bifrons, Wyv. Thoms. Depths of the Sea, (1873) p. 122, 
figs. 17 & 74. 
Plutonaster bifrons, Sladen, Chall. Rep. Ast. (1889) p. 84. 
R=3:27r to 4:87. 
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 madreporiteis not apparent, but its 
position may be easily recognized by the less crowded arrangement 
of the paxillee which hide it. 
Colour creamy yellow. 
R. r 
81 Live 
69 19 
41 12 
29 9 
Distribution. Both sides of North Atlantic, between Faeroe 
Channel and Portugal, and off Massachusetts. 210 to 1360 fms. 
