82 THE VOYAGE OF H.M.S. CHALLENGER. 
No pedicellariz are present. 
Anal aperture subcentral. 
Actinostome nearly closed by the mouth-plates. 
Remarks.—In this genus is included the handsome North-Atlantic form to which Sir 
Wyville Thomson gave the name of Archaster bifrons. It differs in some respects from the 
majority of species which I place in the genus, especially in the character of the adambu- 
lacral armature, and in the-spinulation of the plates of the actinal interradial areas. The 
marginal plates are also more conspicuously armed. In so far, however, as the adam- 
bulacral armature is concerned, it will be found to correspond with the earlier stages of 
the armature in the majority of species of Plutonaster, and the characters of which are 
still exhibited on the plates near the extremity of the ray. After careful study I see 
no reason for separating this Asterid from the other forms which I have grouped together 
under the name of Plutonaster. 
The two starfishes which have long been known under the names of Astropecten 
subinermis, Philippi sp., and Archaster parelii, Diiben and Koren, are in my opinion 
very close allies of the present group of species. They exhibit, however, several constant 
differences which seem to me of a secondary character, and | have therefore placed them 
in a subgenus (Zethyaster) under Plutonaster, to which reference will be made on a 
succeeding page. 
Synopsis of the Species included in the Genus Plutonaster herein described. 
I. Madreporiform body compound, hidden. Adambulacral armature in parallel longi- 
tudinal series: granuliform on the actinal surface. Adambulacral plates long 
and narrow. 
A. Supero-marginal plates with a prominent dorsal spine | 
B. Supero-marginal plates devoid of dorsal spines. 
a, Infero-marginal plates armed with a spine. Rays elongate. 
a. Large marginal plates. Paxille tabulate and compact. No tubercie 
on the supero-marginal plates. Granulation of the marginal = 
plates truncate and obtuse. 
#, Supero-marginal plates broader than the paxillar area. Secon- 
dary adambulacral armature with a well-developed row of 
spinelets ; those external in groups on each cae (two or 
three groups) . : : : marginatus. 
8. Supero-marginal plates narrower than the parillsa area. Secon- 
dary adambulacral armature all granular, subequal, equi- 
distantly spaced, not grouped . 6 : . 5 ; rigidus. 
b, Small marginal plates. Paxille simple, radiating. Conical tubercle 
on the supero-marginal plates. Granulation of marginal plates 
conically pointed . - é : > . ambiguus. 
b. Infero-marginal plates devoid of a spine, or wali W. ith a minute arent in 
old age. Rays short and thick. 
a. Disk broad, rays narrow 
b, Disk small, rays broad 
5 : bifrons. 
3 notatus. 
abbreviatus. 
