502 BULLETIN 100^ UNITED STATES NATIONAL MUSEUM. 



bulacral plates ; disk finely spinulate, a few spinelets extending upon 

 ray; interbrachial skeleton resembling that of Brisingella, but the 

 first marginals unequal in size ; first adambulacral plates entirely sep- 

 arated ; adambulacral armature very simple. Genotype, Hymeno- 



discus agassisi Perrier Hymenodiscus Peri-ier/ 



<f Rays composed of ambulacral and adambulacral plates and an abactinal 

 integument devoid of skeleton, but covered with numerous, relatively 

 large pedicellariae arranged in transverse bands. Disk not known. 



Genotype, Oymnohrisinga sarsii Studer Oymnobrisinga Studer. 



b'. Abactinal skeleton present on disk, and on proximal portion of rays at 

 least, either in the form of transverse independent skeletal arches or 

 as a continuous covering of thin plates, or both arches and intervening 

 thin plates. 

 c\ Disk plates large, with a comparatively few, fairly large, disk spines, 

 and a vertical series of about 4 conspicuous, lateral spines to each 

 successive skeletal arch of the rays, these lateral combs occurring at 

 regular intervals all along side of ray ; mouth plates expanded fan- 

 wise toward actinostome, nearly closing entrance to ambulacral fur- 

 row ; subambulacral spine with a modified truncate tip ; first pair of 

 adambulacral plates of each interbrachial angle united, but the 

 united pair of conspicuous first marginal plates is interpolated from 

 above between their distal ends; syzygy (?); related to Odinia. 



Genotype, Brisingasicr rohillardi de Loriol Brisingaster de Loriol. 



c^ Disk plates small, bearing 1 to several small spinelets; on the ray 



only 1 lateral or marginal spine corresponding to the variably spaced 



inferomarginal plates; never a vertical comb of conspicuous lateral 



spines. 



d^ First and second adambulacral plates as well as the upper part of the 



second and third ambulacral plates united by syzygy (nonmuscular 



symphysis). 



^ For description see Perrier, 1884, p. 189, pis. 1 and 2. The type of Hymenodiscua 

 agassizi (No. 1448, Museum Comparative Zoology) is almost certainly a very immature 

 specimen, as it is small, and there are no gonads. There are no skeletal arches on the 

 rays and the greater part of the thin abactinal integument has been removed. The fine 

 spinulation of the disk extends upon the base of the ray. The abactinal integument of 

 the ray, although very delicate, contains a single layer of latticework holothuroid plates, 

 some of which at the very base of the ray bear minute spinelets. From this it would 

 seem that the abactinal wall of the ray is destined to be similar to that of Freyella, 

 unless in the fully adult animals the plates retain their embryonic character. 



The interbrachium resembles that of Brisingella, but differs in having the first mar- 

 ginals (those which bound the apex of the interbrachial angle) unequal in size. (See 

 Fisher 1918, p. 104, figs. 1 and 2.) In Brisingella these plates arc equal, and the suture 

 between the interradlal ends is on a line with the interradial, or median oral, suture. 

 There is a distinct syzygy between the first and seconcj adambulacral plates. The inter- 

 brachia are not so open as in Brisingella, as the inner ends of the first adambulacral plates 

 are normally In contact, or very nearly so. In an adult specimen we would expect to 

 find these plates still closer together. It is worth noting that in Freyellaster and in 

 Brisinga, s. s., the first marginal plates are of unequal size. (See pi. 156, figs. 1 and la.) 

 Yet In its present juvenile form the interbrachial angle is different from that of either 

 Freyellaster or Brisinga, while the entire absence of costal arches, as well as of gonads, 

 may reasonably be attributed to immaturity. It does not seem possible to identify this 

 problematical form with any other genus, except the even less known Oymnohrisinga of 

 studer. 



Gymnohrisinga sarsii (Abhandl. Akad Wiss. Berlin, Anhang, Abth. 2, 1884, p. 13, pi. 3, 

 fig. 5) is based upon a brisingoUl ray only. This lacks a dorsal skeleton, and while the 

 large podiccllaria figured by Studer is different from those of Hymenodiscus agassizi, I am 

 quite unprepared to offer an opinion as to the generic distinctness of the two species. 



