82 THE VOYAGE OF H.M.S. CHALLENGER. 



No pedicellariae are present. 



Anal aperture subcentral. 



Actinostorne 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 I have therefore placed them 

 in a subgenus (Tethyaster) under Plutonaster, to which reference will be made on a 

 succeeding page. 



Synopsis of the Species included in the Genus Plutonaster herein described. 



I. Madreporiforrn body compound, hidden. Adambulacral armature in parallel longi- 

 tudinal series : granulif orm on the actinal surface. Adambulacral plates long 

 and narrow. 



A. Supero-marginal plates 'with a prominent dorsal spine ..... bifrons. 



B. Supero-marginal plates devoid of dorsal spines. 



a. Infero-marginal plates armed with a spine. Eays elongate. 



a. Large marginal plates. Paxillse tabulate and compact. No tubercle 

 on the supero-marginal plates. Granulation of the marginal 

 plates truncate and obtuse. 

 a. Supero-marginal plates broader than the pasillar area. Secon- 

 dary adambulacral armature with a well-developed row of 

 spiuelets ; those external in groups on each plate (two or 

 three groups) ......... marginatus. 



/3. Supero-marginal plates narrower than the paxillar area. Secon- 

 dary adambulacral armature all granular, subequal, equi- 

 distantly spaced, not grouped ...... rigidus. 



b. Small marginal plates. Paxillse simple, radiating. Conical tubercle 

 on the supero-marginal plates. Granulation of marginal plates 

 conically pointed ......... ambiguus. 



b. Infero-marginal plates devoid of a spine, or only with a minute rudiment in 

 old age. Eays short and thick. 



a. Disk broad, rays narrow ........ notatus. 



b. Disk small, rays broad ......... abbreviatus. 



