96 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



in general appearance, but differing in having no median radial area 

 of smaller irregularly arranged paxillae; in having the paxillae in 

 regular curved transverse series on rays, with fewer paxillae to the 

 series than in hawaiiensis ; in having finer superomarginal spinelets 

 and more numerous minute superomarginal fasciculate pedicellariae ; 

 in having slenderer and fewer inferomarginal spines, arranged in 2 

 oblique arcuate lateral series, and 1 transverse actinal series (not 

 3 and 2, respectively, as in fiawaiiensis) ; in having tiny inferomar- 

 ginal and numerous prominent actinal intermediate and subambu- 

 lacral fasciculate pedicellariae ; in having fewer subambulacral spines 

 and prominent central spines to the actinal interradial plates. 



Description. — The general form and proportions are like those of 

 hawaiiensis and the abactinal surface has much the same appear- 

 ance, except that the paxillae are more regularly and compactly 

 arranged, and along the median line of the ray there is no well- 

 defined stripe of small and irregularly arranged paxillae, as shown 

 in plate 5, figure 2, Fisher, 1906. Instead the median radial paxillae 

 are distinctly larger and fewer, and the paxillae of rays form slightly 

 curved transverse rows, which continue uninterrupted across the 

 radial area. At the base of the ray about 5 of these correspond to 

 2 superomarginals, and opposite the fourth superomarginal a trans- 

 verse series contains about 16 paxillae, while in hawaiiensis there are 

 about 22. The paxillae of disk are larger than those of the rays, 

 and, as in hawaiiensis, are very close set. The general form of the 

 paxillae is alike in the two species, but in diploctenius the spinelets 

 are more delicate. A typical paxilla from the proximal portion of 

 ray has 12 to 15 peripheral and 5 to 7 central, terete, round-tipped or 

 subcapitate spinelets, while a large paxilla from the interradial re- 

 gions of disk has 20 peripheral and 15 spaced central spinelets. 

 Many of the paxillae bear a small central fasciculate pedicellaria 

 with 4 or 5 practically unmodified, paxillar spinelets for jaws. They 

 surround a hole in the tabulum, I have not been able to find them 

 in hawaiiensis. 



The abactinal plates, or bases of paxillae, as seen from the coelomic 

 surface, are much longer than wide and very regularly arranged in 

 transverse series. The shape, arrangement, and size are shown by 

 the fiffure (pi. 16. fig. 2). Unlike hawaiiensis, diploctenius has over 

 most of the ray a very regular, median radial series of elongate 

 plates, accompanied by very small secondary plates, which externally 

 have tabula and spinelets but are quite inconspicuous between the 

 large paxillae and do not interrupt their regularity. Sometimes 2 

 consecutive radial plates are separated by a small secondary plate. 

 Papulae are absent from this radial stripe and center of disk; else- 

 where they occur very regularly in sixes about each plate. 



