10 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



prominent spines. The mouth plates arc small (but relatively larger in medium- 

 sized and young examples) and the strongly arched sutural surface is turned toward 

 the actinostome so thai the chord or long axis of the surface is not horizontal but 

 e nearly vertical, varying to about 45° in younger specimens. There are two 

 spines on each plate a slightly curved tapering blunt one at the inner end, directed 

 over actinostome, and a straight tapering longer one, about the middle, and bent 

 away from actinostome. The first adamimlacral plate is conspicuously larger than 

 the second, and the second than the third. In the largest specimen the first plate is 

 actinally about three-fourths as long as the corresponding sutural dimension of the 

 mouth plates; the second is three-fourths the first; the third, three-fourths the second, 

 and the fourth, three-fourths the third; the fifth and succeeding plates about the same 

 as the fourth. Viewed from the furrow face these plates resemble the stones of an 

 arch, as the series is bent toward the actinostome at the inner ends. In small speci- 

 mens this arching becomes less and less evident with decrease in size, and fewer 

 plates are joined behind the mouth plates. The first and second adambulacrals, 

 and sometimes the third also, carry but one spine. The first furrow spine may occur 

 on the third plate or not until the seventh or eighth. 



Abundant abactinal, intermarginal, and actinal papulae, the number increasing 

 greatly with age. 



Pedicellariae very abundant. Small ovate to lanceolate straight pedicellariae, 

 much smaller than the spinelets, are scattered over the abactinal surface (largest in 

 interradial depressions), on the supermarginal, intermarginal, and inferomarginal 

 plates and the bases of the spines, and on the adambulacral and mouth plates and 

 spines. There is a cluster of about 5 to 10 near the end of the furrow spines, and in 

 addition to the more numerous crossed pedicellariae, also several near the end of the 

 other adambulacral spines. Straight pedicellariae of the actinal surface are fully 

 twice as large as the dorsal; of the former the intermarginal are probably the largest, 

 though in some specimens the size increases slightly toward the furrow (but the 

 adambulacral spinal pedicellariae are smaller). The number of pedicellariae varies 

 greatly with age, and to some extent with locality. 



Small crossed pedicellariae present on the abactinal and actinal surfaces, forming 

 usually a single circle around the dorsal spines, and are distinguishable in dried speci- 

 mens from the straight pedicellariae by their smaller size and blunt rounded tips. 

 (The straight pedicellariae are found on the plates and among the papulae, and some 

 of them are quite as small as the crossed.) The crossed pedicellariae, also, form 

 clusters on the upper and outer sides of all the marginal and subambulacral spines, 

 but not on the alternate jurrow spines which carry only straight pedicellariae. 



The pedicellariae are treated in some detail under Variations, where comparative 

 measurements are given. The measurements of pedicellariae of a giant specimen 

 would be somewhat misleading. See also, figures. 



Madreporic body large, circular, nearly flat to decidedly convex, situated one- 

 half r from center. It is 7 mm. in diameter and the fine striae radiate from the center. 



Variatiojhs.— The breadth of the ray and width of intermarginal and actinal 

 channels increases with age, as do, in general, the number of spines, pedicellariae, 

 and papulae, the rigidity of the skeleton, and thickness of the spines. These changes 



