1 50 BULLETIN" 100, UNITED STATES NATIONAL MUSEUM. 



Subambulacriil spines about 20. Subanihulucral spines 25 to 35. usu- 



ally 25 to 30. 

 Mouth plates where denuded with a Mouth plates where denuded with a 

 broad rounded inner end to the com- narrow inner end to the combined pair 

 bined pair and no appreciable angle and a longer spine bearing furrow 

 between the outer ends ; spine bearing margin ; a conspicuous deep angle be- 

 margin adjacent to furrow, shorter. tween the diverging outer ends of the 



plates. 



Dipsacaster pentagonalis Alcock. also from the Andaman Sea, 

 differs in having conspicuous inferomarginalspinules and an enlarged 

 subambulacral spine. 



Dipsacaster magni-ficus (H. L. Clark) ^ is probably closely related 

 to D. imperialis. Clark's type is of about the same size as that of 

 imperialis, the measurements being R, 155 mm., r. 50 mm., R=3 r, 

 breadth 45 to 50 mm. 



The species of Dipsacaster lack large spines or other prominent 

 tangible features for comparison, so that distinct forms may have 

 a close superficial resemblance. This is the case with D. magnifictts. 

 In general appearance it greatly resembles imperialis, but judging 

 from Clark's careful description the following differences exist: 



Although magnifiGUs is slightly smaller its madreporic body is 



2 mm. broader, carries 50 instead of 25 paxillae on its surface, and 

 is 9 instead of 14 mm. from the inner margin of the superomarginal 

 plates. The latter are 46 or 47 instead of 39 or 40 in number. They 

 are thus smaller than in imperialis. Of the inferomargmals Clark 

 says : " Inf eromarginals correspond in number and position with the 

 superomarginals, but are much larger and project conspicuously 

 beyond them; the interradial oites are 2 mm. long by 9 mm. wide 

 (8 mm. wide in imperialis) , while those near the middle of the arm 

 are 3 mm. by 6 mm. (4 by 5.2 in imperialis) ; their covering consists 

 of a close coat of short, flattened, blunt spineiets, much coarser than 

 those on the superomarginals." In imperialis the spineiets are short, 

 sharp, subconical, slightly squamiform, in general not much larger 

 than the superomarginal spineiets except for a tuft on the aboral 

 tumidity of each plate, these tufts giving a serrate appearance to 

 the ambitus. Clark does not mention these enlarged spinules. and 

 they are not discernible in the figures. The paxillae are a little 

 larger than in magni-ficus, the glomerular tuft at the top being 1.5 

 mm. in diameter on the disk, where the spineiets are compact. There 

 are 30 or 31 oblique transverse rows of paxillae meeting the first 10 

 superomarginals, or 3 to a plate (usually 2, but sometimes 3 in 

 magniflcus) . In imperialis the oblique transverse series of plates 

 on either side of the midradial line are very distinct. The plates 



1 Lonchotaster magniflcus Clark, 1916, p. 30, pi. 6, figs. 1 and 2 ; Great Australian 

 Fight, 80-120 fathoms. 



