14 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



or three, delicate, sharp, spinelets 0.5 to 0.75 mm. long and sheathed in delicate mem- 

 brane. These groups of spinelets are spaced about 1 mm. apart in the center of 

 disk, and near the border, about half that distance, or even less. Madreporic body 

 prominent, on edge of disk. On its adcentral side is a group of plates (eight or nine) 

 which are tightly fitted together, and combined are about as large as the madreporite. 

 Interradial plate small, broader below than above (somewhat oval in form) the sur- 

 face subplane sometimes with a shallow groove in the middle of the upper half. The 

 lower end meets the inner ends of the interradial pair of marginal plates. (See pi. 2, 

 fig. 2.) At the upper end of the interradial plate is an irregular thick movable plate 

 in the abactinal integument. It may bear a few spinelets and is much larger and 

 thicker than the other abactinal plates. No pedicellariae on disk. 



Rays fragile, very deciduous, the abactinal membrane thin and devoid of integu- 

 mentary prickles. Costal ridges delicate, widely spaced (opposite alternate adam- 

 bulacral plates), narrow, irregular, prominent, composed of elongate plates which 

 imbricate by their ends. These plates in the proximal portion of the genital area, 

 at least, bear one or two rather stout, subconical prickles, and in life each costal 

 ridge is overlaid by a cushion of pedicellariae. Between the costae, and correspond- 

 ing to the alternate adambulacral plates, is a transverse prominent saccular band 

 of pedicellariae. These are continued throughout the ray, and after the costae have 

 ceased they occur opposite about every adambulacral plate. The costae number 9 

 to 11, possibly 12 in some cases, and they extend 4.5 r along the ray, or only one- 

 fourth total length of the ray. 



Lateral spines attached to the rather prominent lowermost plate of the costae, 

 and hence at the upper edge of alternate adambulacral plates. They are very fragile 

 and it is difficult to find a complete one. At the middle of the ray of a specimen 

 from station 4387 the spine is 14.5 mm. long ( = 7 adambulacral plates) and one at 

 the middle of the costal area is 9 mm. long ( = 4 adambulacral plates). 



Adambulacral plates comparatively slender, longer than broad, and at base of 

 ray fully twice as long as the height seen from the side. Furrow margin deeply 

 excavated. The armature consists of a very delicate furrow spinelet at the adoral 

 end of plate, surmounting a slight boss and armed with a terminal pad of minute 

 pedicellariae; and on the actinal surface of the distal half of the plate is a delicate 

 subambulacral spine about 5 mm. long covered in life by a rather thick sheath closely 

 beset with pedicellariae. This spine is shorter on the plates opposite which there is 

 a lateral spine. The small furrow spinelet continues to very near the tip of ray. 

 On the outer attenuate portion of ray the adambulacral plates are, of course, slenderer 

 than proximally, and the furrow spinelet is spaced about one-third the length of the 

 plate from the proximal end. 



The first adambulacral is joined to the second by a nonmuscular articulation. 

 The ray breaks at this point. The first adambulacral plate is separated from that 

 of the adjacent ray by the outer ends of the combined mouth plates. The inter- 

 brachial angle is formed by the first marginal plates of each ray, which join by their 

 inner ends, and form a reversed Y with the interradial plate. The first marginal 

 lies along the upper edge of the first adambulacral; the second along the upper border 

 of the second adambulacral, while there is a small third marginal above the third 

 adambulacral, and the first costal arch commonly occurs at the distal end of the 



