100 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Genus PSILASTER Sladen. 



Psilaster Sladen, Narr. Chall. Exp., vol. 1, 18S5, p. 611. Type, Astropecten 



andromeda Miiller and Troscbel. 

 Phidiaster Koehler, 1909, p. 28. Type, Phidiaster ayassisi Koehlei-. 



PSILASTER GOTOI Fisher. 



Plate 16, figs. 3, 3a-h ; plate 20, fig. 3 ; plate 21, fig. 1 ; plate 30, fig. 4 ; plate 35, 



fig. 3. 



Psilaste7- yotoi Fisher, 1913a, p. 609. 



Diagnosis. — Rays 5. E,=58 mm., r=14 mm., R=i.l r; breadth 

 of ray at middle of interbrachiiim, 16 mm. ; at third siiperomarginal. 

 13.5 mm.; height of combined marginal plates at middle of inter- 

 brachium,T.5 mm.; at middle of fourth superomarginal, 6.5 mm. Disk 

 fairly large, rays evenly tapered to a pointed extremity ; interbrachia 

 about 90°, rounded; superomarginals Avith a single row of prominent 

 appressed spines extending nearly to tip of ray; inferomarginals 

 with 2 series, one near the upper end of the plates and the other 

 near the lower; adambulacral plates with 7 or 8 furrow spines 

 proximally, and about 10 subambulacral spines in 2 series; mouth 

 plates with horizontal fan of 4 prominent teeth. Differs from 

 Ps. agassizi Koehler in having only 1 superomarginal series of 

 spines, no well-defined central naked area on inferomarginals, and 

 in having the superomarginals less conspicuous dorsally, and the 

 inferomarginals practically confined to the sloping side wall of ray; 

 furrow spines slightly more numerous. 



D escri-ption. — Paxillae in slightly oblique transverse series, meet- 

 ing a circular area (extending as far as madreporic body) of disk 

 and a narrow radial area of the proximal half of ray where the 

 plates are without regularity and closer together, owing to the ab- 

 sence there of papulae. The largest paxillae are on the outer part 

 of Ihis circular area and its stellate radial extensions, and they 

 decrease in size toward the center of disk and margin and along 

 the rays both toward margin and extremit3\ In center of disk is an 

 epipi-octal cone with minute paxillae, and a tiny anal aperture. 

 At base of ray 6 transverse rows of paxillae correspond to 2 supero- 

 marginaJs, and between the second and third superomarginals about 

 21 paxillae can be counted across the ray to a corresponding point 

 QVi tiiC other side. The paxillae are flat-topped and the largest 

 contain 15 to 20 cylindrical, very truncate spinelets, jammed to- 

 gether so that there is no clear distinction between a peripheral 

 series and central group. Some of the peripheral spinelets are here 

 and there notably slenderer, especially in a smaller specimen from 

 station 5470, where the spinelets are not so crowded. A paxilla 

 at base of ray has 8 to 18 spinelets according to whether it is near 

 the margin or radius: on the outer fourth of ray, 5 to 8 spinelets. 



