440 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



tinal surface. At base of ray there are 1 or 2 extra short intermediate 

 series of plates and 1 short intermarginal series, sometimes extending 

 a third of the length of ray. 



On the furrow face of the adambulacral plates, which are slightly 

 wider than long, is a vertical series of 3 or 2 compressed spinelets. 

 On the actinal surface are about 8 or 9 prominent spines of which 3 

 form, usually, a triangle on the furrow jnargin, 2 slightly shorter 

 stand back of them, and 3 or 4 decidedly shorter, but much longer 

 than the actinal intermediate spinelets form a group on the outer 

 end of plate. The prominent spines are tapering, pointed, and about 

 2 plates in length. The subambulacral spines form a bristling arma- 

 ture along the borders of the furrow, notable by their superior length 

 over all the other spines. 



The madreporic body is slightly convex, small, and situated at 

 about the middle of r. The striae are few, transverse, and bear tiny 

 spinelets, usually in a single series on the tup of the ridge. 



Type.— C?it. No. 37067, U.S.N.M. 



Type-locality. — Station 5536, between Negros and Siquijor, 279 

 fathoms, green mud, bottom temperature 53.5° F. ; 3 specimens. 



Distribution. — Central part of the Philippine group, 279 to 530 

 fathoms. 



Specimens examined. — Four ; besides the types, 1 from station 5219, 

 between Marinduque and Luzon, 530 fathoms, green mud, bottom 

 temperature 50.8° F. (typical). 



Remarks. — This slender-rayed form differs from H. tnutans Koeh- 

 ler in having w-ell-defined marginal plates, and a vertical series of 

 usually 3 spinelets on the furrow face of the adambulacral plate. The 

 lowest of these 3 spinelets is close to the end of the ambulacral plate. 

 I am uncertain whether mutans has more than the 1 furrow spine- 

 let. In the description, the plates are said to have " 3 spines on the 

 inner edge directed into the furrow." The furrow spinelets of 

 arcystata are in a distinct vertical series and not on the furrow mar- 

 gin as generally understood. In mutans the actinal plates are de- 

 scribed and figured as being arranged without order. II. mutans 

 was dredged near the Andaman Islands in 480 fathoms. 



From H. praestans (Sladen), arcystata differs in having the 3 

 furrow spinelets, slenderer raj's, less compactly placed actinal plates 

 with larger actinal papular areas, and longer spines bordering the 

 furrows. This species was taken off the Crozet Islands in 210 

 fathoms, and is a subantarctic form. 



