174 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



be the more prominent columns. Or the entire abactinal area of the 

 rays may be composed of ossicles arranged in columns and in 

 quincunx when the plates are of nearly equal size. The number of 

 columns varies with age and in different species, the greatest number 

 observed being 13 at the base of a ray. 



Ambital plates often not readily distinguishable at maturity from 

 the radial and supramarginal columns. The growing distal tips of 

 the rays of Urasterella have no ambital plates, but shortly after their 

 introduction they begin to develop the rodlike extension so charac- 

 teristic of this genus. Of these columns there may be as many as 

 four at the base of a ray on each side of the supramarginals. 



Inframarginals, as such, usually not distinguishable at maturity 

 from the ambital plates. Their position is rather abactinal than 

 actinal. At the distal end of rays, however, these plates lose their 

 ambital character, passing over somewhat to the actinal side, and 

 closely adjoin the adambulacrals. In the young of U. ulrichi the 

 inframarginals are well developed in the axillary regions and here 

 there is also a single interbrachial marginal plate. More dis tally, 

 however, the inframarginals have the character of the ambital plates. 

 In none of the mature specimens have been seen well-developed or 

 larger axillary inframarginals or interbrachial marginal plates. In 

 U. asperula the axils have actinally single plates which may be 

 remnant inframarginals. 



Madreporite abactinal, of medium size, slightly convex, depressed 

 and granules triate. 



Adambulacral plates very numerous, coin-shaped, and arranged on 

 edge with the actinal surface pustulose. Each plate on its actinal 

 surface bears two or three short, thick spines, and on its ambulacral 

 side there is another similar spine. Along the outer edge of these 

 plates toward the abactinal side there is another row of spines, in 

 pairs, which are long and slender, flat, and longitudinally grooved 

 on two sides. The adambulacral columns terminate in small tri- 

 angular plates of the oral armature. In the young of U. ulrichi five 

 very stout, short, pointed spines (tori) are inserted inside of the plates 

 of the oral armature. 



Ambulacral columns with the ossicles generally slightly alternating, 

 but they may also be opposed, one plate to each adambulacral piece, 

 more or less wedge- or club-shaped, and those of each column par- 

 tially overlapping. Medially the columns loosely adjoin. Each 

 plate is excavated laterally, along the proximal edge, leaving a more 

 or less long, slender podial opening between adjoining plates. 



Genoholotype. Uraster ruihveni Forbes. The above diagnosis, 

 however, is based on Stenaster grandis Meek, a form thought to be 

 in generic harmony with the genotype. 



