92 DISCOVERY REPORTS 



Distribution. Antarctic, circumpolar. Kerguelen, 127 fathoms, volcanic mud 

 (Sladen). South Shetlands, 18-420 m., mud; AdeUe Land, 151 fathoms; Shag Rock 

 Bank, 160 m., gravel, sand; South Georgia, 250-310 m., clay (Koehler). 



South Georgia, 91-270 m. ; South Orkneys, 244-344 m. ; Palmer Islands, 160-500 m. ; 

 Bouvet Island, 357-377 m.; Ross Sea, 351 m. (' Discover)'). 



Genus Psilaster Sladen 



Psilaster Sladen, Narr. Cliall. Exp., i, 1885, p. 611. Type Astropecten andromeda M. & T. 

 Ripaster Koehler, 1906, p. 4. Type R. charcoti Koehler. 

 Phidiaster Koehler, 1909, p. 28. Type Ph. agassizi Koehler. 



Remarks on Ripaster. There are enough young specimens to give a hint as to the 

 relationship of the genus Ripaster since they are more "generalized" than the adults. 

 The smallest has R 5-5 mm., r 2-5 mm. Up to a size where R is 16-20 mm., and R 

 equal to about 3 r, they resemble a Leptychaster with a vertical comb of 3 or 4 appressed 

 inferomarginal spines. The superomarginal spines are just beginning to differentiate 

 from the other spinelets of the plate and would not be significant if we did not know the 

 end result. No Leptychaster, of course, has specialized marginal spines. As a conse- 

 quence such a sea star would probably be allocated to Psilaster, rather than to Perse- 

 phonaster especially if the mouth and adambulacral plates were carefully examined. 



Koehler compared Ripaster with Dytaster. It has nothing to do with that rather 

 specialized and easily recognized genus. Ripaster in general appearance closely re- 

 sembles a Persepho7iaster of the broad-rayed type, such as P. euryactis of the Philippines. 

 This resemblance concerns especially the form of ray, its attenuation at extremity and 

 the bristling marginal armature. Like Persephonaster, Ripaster has the gonads confined 

 to the interradial region and lacks, in the actinal interradial area, an unpaired series of 

 plates extending from the outer end of the mouth-plates nearly or quite to the infero- 

 marginals. Such an unpaired series, more or less well developed, is present in Lepty- 

 chaster, Blakiaster and Astromesites. 



The adambulacral and mouth-plates of Ripaster are like those of Psilaster rather than 

 Persephonaster. The first adambulacral is strongly compressed as in Psilaster and Bathy- 

 biaster, with 2 closely appressed transverse rows of spines, 2 or 3 of which may be 

 allocated to the narrow, angular, furrow margin. Similarly, each mouth-plate carries 

 2 closely appressed rows of spines extending from outer to inner end of each plate, as in 

 Psilaster and Bathyhiaster. In Persephonaster the first adambulacral is nearly or quite 

 like the others, with a furrow comb of 5 or more spines. In Ripaster, Psilaster and 

 Bathyhiaster the second adambulacral is also compressed, and even the third shows some 

 distortion in Bathyhiaster and Psilaster. The mouth-plates of Persephonaster have a 

 more extensive actinal surface, with numerous intermediate suboral spines. There is a 

 definite marginal comb bordering on the abulacral furrow (not differentiated in Ripaster, 

 Psilaster, Bathyhiaster). In Persephofiaster there is a very characteristic angular group or 

 tuft of small spines at the inner end of each mouth-plate, above the tooth and the obvious 

 marginal spines. These spines^ are adjacent to the first tube foot, and the upper spinelets 



1 Fisher, 19 19, pi. 38, fig. i. 



