224 BULLETIN 7C, UNITED STATES NATIONAL MUSEUM. 



rouiuled at its base by about eight to fifteen ratliating, very small, papillifomi 

 spinelets; the numerous smaller paxillos, scattered between, with from one to eight 

 equal, upright or slightly radiating similar spinelets, one often standing in the 

 center of a'^circular group. Individual paxilhe clearly visible to naked eye; said 

 to be invisible in mimicus. The larger paxilla^ with a central spinule extend entire 

 width of arm and to its tip. Scattered all over abactinal surface are numerous pedi- 

 cellarias formed of two to six or seven thickened spinelets, about intermediate in 

 length between the central spinule and smaller spinelets. The spinelets of pedi- 

 cellariiB are tliicker than any others of abactinal surface, and form either a coor- 

 dinate (''fascicular") group, or two opposing combs, usually two plates entering 

 into the formation of a pedicellaria. Pedicellarise are found on both large and 

 small i)lates, are largest on disk, and extend to end of ray in some specimens; in 

 others they are comparatively scarce, and lacking on outer part of ray ; in a small 

 number of examples only four or six are present, on disk. The typical form from the 

 Galapagos to Gulf of California has no abactinal pedicellariae. 



Papularium broadly elliptical, convex, so situated that a line drawn across 

 base of ray at interradiai angle bisects it. It is about as long as half the width of 

 ray at this point. Papulie mcrease in number with age; in large specimens, forty 

 to seventy-five to each area. The abactinal wall is much thicker on the papular 

 areas, since the plates are here in the form of vertical rods bearing a small crown 

 of spinelets externally (two to six or seven, with sometimes a central spinule of 

 inconsf)icuous size). The rods are compressed and oriented so as to appear to 

 radiate from larger central plates as shown in the figure (pi. 57, fig. 1). On 

 account of the form of the plates of papularium that area is convex also on the 

 inner or coelomic side of the body wall. Its plates are rather closely boimd 

 together by tissue. Elsewhere the plates are thm, subcircular, and distinctly 

 spaced, the difference in size between the large and small plates being very 

 noticeable. 



Superomarginal plates, thu'ty-fom* in an example with 11 = 72 mm., confined to 

 side wall of ray, much longer than high on outer part of ray, where they are nearly 

 opposite the inferomarginals. Proximally, however, the plates are nearly or quite 

 as high as long and are alternate with the inferomarginals. Sm^'ace of plates, 

 sometimes with exception of a small area beneath the spine, is covered with spaced 

 short papillifomi spinelets. Each plate bears near middle of upper edge a needle- 

 Uke sharp spine, the fourth from interradius longest (about 4 mm.). The first 

 four or five plates have below this an accessory lateral spine, wliich in the first three 

 or four plates is nearly or quite as long as the primary spine. A variable niunber 

 of the superomarginals, beginning with the fourth to tenth, have a small pedicellaria 

 composed of two to four stubby spinelets standing usually over the mtermarginal 

 suture, some of tlie spinelets, therefore, on the inferomarginals. The number of 

 pedicellaria} is variable; sometimes only a few distal plates have them, at others 

 fully two-thirds of the plates. 



Besides a lateral prominent primary spine, slightly longer than those of superior 

 series, the first five or six plates bear two to four unequal accessory spinulcs on the 

 inner part of the plate; then beyond these a single spinule to outer thii-d of ray. 



