184 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



R = 6.5r. Width of ray at base in the different individuals 4, 7.5, 

 and 9 mm., respectively. 



Rays short and tapering rapidly when young but at maturity very 

 long, highly convex abactinally, and tapering very slowly. Disk 

 comparatively large in the young but small at maturity and formed 

 by the united ends of rays. 



Abactinal area of disk and rays composed of numerous small, tumid, 

 or highly spinose plates which in the rays are arranged distinctly 

 in columns and in quincunx. In the angles between all of the 

 plates are left subcircular small openings. On the rays medially 

 there are two columns of large convex plates, variously stellate and 

 not produced into short, blunt rods. These are the supramarginals, 

 and between them are a few widely separated plates of the radial 

 columns. In the young the two proximal radial plates are sepa- 

 rated from each other by from one to three supramarginals, while 

 the third plate is separated from the second by three or four supra- 

 marginals. In a mature specimen there appear to be about eight 

 of these radial plates on each ray and distally they are separated 

 from one another by five supramarginal ossicles. On each side of the 

 supramarginal columns are from one (distally) to five (proximally) 

 columns of smaUer or accessory ambital plates. In a full-grown 

 specimen there are 11 to 12 in 10 mm., triangular in outline, and 

 drawn out into short, stout, blunt rods. In the center of the disk 

 in the youngest specimen (probably also at maturity) there is a 

 single large depressed plate around which is a first circle with 6 

 plates followed by a second having 16 pieces. Of the latter, 5 are 

 basal radials, 10 basal supramarginals, and 1 the madreporite. 

 All of the disk plates are variously stellate, and are the largest plates 

 of the abactinal area, the size gradually diminishing distally along 

 the rays. 



Madreporite concentrically striate, somewhat smaller than the basal 

 supramarginals, between two of which it is placed. 



Adambulacral plates seen only in the young specimen, very strong, 

 carinated, subquadrate, wider than long, and about 19 or 20 in each 

 column 10 mm. long, not counting the oral armature pieces, which are 

 subtriangular in outline. Inside of each pair of oral armature plates 

 is placed a very stout but short subcircular spine which has not been 

 observed in other species. No other spines are preserved. 



Ambulacral furrows very narrow and deep, concealing the ambu- 

 lacral plates. 



In the axils of the youngest specimen, outside of the adambulacrals, 

 are situated three prominent plates, the proximal one being the 

 smallest. On each side of these along the ray is a single column of 

 plates which rapidly diminish in size and separate with increasing 

 interspaces distally. Their number can not be made out, but they 

 appear to continue to the tips of the rays, where plates occur that 



