192 BULLETIN 1G, UNITED STATES NATIONAL MUSEUM. 



PSEUDARCHASTER DISSONUS Fisher, 

 n. 34, figs. 1-3; pi. 57, figa. 7, 7a. 

 Pseudarchaster dissonus Fisher, Zool. Anz., vol. 35, March 29, 1910, p. 551. 



Diagnosis. — Closely related to Ps. pectinifer Ludwig, but differs in having 

 actinal hivalved |)eclicellaria? of a highly characteristic form, coarser granules to 

 abactinal paxilla? and superomarginal plates, proxLnially smaller adamhulacral 

 plates, fewer actinal adamhulacral spinelets, and pointed furrow spines. R = 170 

 mm.; r = 61 mm.: R=2.G r. Breadth of ray at interradial line, 72 mm.; at one- 

 thu-d R from center, 50 mm.; at two-thirds R, 24 mm. Interbrachial arcs wide and 

 rounded; rays tapering, with straight sides, pointed; abactinal area sunken on 

 interradii and capable of inflation. Abactinal paxillse large, with fifteen to twent}'- 

 five central, and about the same number of slender peripheral granules; abactinal 

 plates subcircular. Marguial phites narrow, tumid, the two series difficult to differ- 

 entiate; covered with thickened papilliform spinelets or elongate granules on upper 

 series and spinelets on lower; two to four or five enlarged spinules on inferomar- 

 ginals. Adamhulacral plates longer than broad, with very acute furrow margin 

 bearing sLx to eight spinelets, and on actinal surface three to five slender spinelets 

 and one or two enlarged sharp spinules; all spinelets sheathed in membrane. Acti- 

 nal intermediate areas very extensive; plates covered with slender, skin-sheathed 

 spinelets; each plate with one, sometimes two, enlarged spinules, all directed 

 toward margin. On basal third of ray, valvate pedicellarias on transverse sutures 

 between first longitudinal row of actinal intermediate plates; or valvate pectinate. 

 Well-developed superambulacral ossicles, lodged in membranous septa running 

 from ambulacrals to inferomarginals. 



Description. — Paxillaj large; largest on proximal radial areas, decreasing in size 

 toward marginal plates and radially toward extremity of ray; in giant specimen less 

 obviously toward center of disk. Paxillar crowns hexagonal; pedicel of paxilla 

 broad and low, widemng at summit; this tabulum, in large paxillse, covered with 

 fifteen to twenty-five central shghtly spaced, rouncUsh or subhexagonal flat-topped 

 granules, quite robust, and variable in shape, but usually low and subcyUndrical. 

 and a peripheral series of fifteen to twenty-five slenderer peripheral granules. 

 Toward margin the spinelets are much fewer and form a convex more compact 

 group. Three longitudinal series touch terminal plate. 



Abactinal jilates subcircular, arranged in longitudmal series parallel with the 

 radial, except on center of disk and extreme margin of area, where there is no regular 

 arrangement. On the latter area the plates are often more or less elongate longi- 

 tudinalh' and in median radial area there are slight indications of lobing. Papulse 

 single, and sLx about each plate. They are distributed all over the abactinal area 

 except for about 8 mm. at tip of ray. 



Superomarginal plates, fifty-six to a ray, are relatively small, and do not 

 encroach conspicuously on abactinal area. Each plate is distmctly tumid when 

 the sjjinelets are cleaned off. Sutural lines very indistinct, it being impossible to 

 distinguish the suture between the two marginal series without first cleaning plates. 

 (In slightly smaller specimen (station 3788) this line can be seen readilv.) The two 



