448 PROCEEDINGS OF THE NATIONAL MUSEUM vol.90 



genera having predominantly 5- and 6-rayed species such as Dipla- 

 sterias, Lysasterias, and Cryptasterias. 



The new genus, which may be called Plazaster, has crossed pedicel- 

 lariae entirely diJfferent from those of Labidiaster, best understood 

 by a comparison of figures. The pedicellariae of Labidiaster annulatus 

 may reach enormous size. Figure 2, plate 67, if enlarged as much as 

 figure 1, plate 66, would occupy the entire plate. They occur thickly 

 on transverse cushions of tissue, as illustrated by Sladen (1889, pi. 

 108) ; and while those of radiosus are somewhat smaller they are still 

 very large and of the same type as in annulatus. The shank teeth are 

 relatively large and occur in a single series. Those of Plazaster are 

 small and in several series. In Labidiaster much smaller pedicellariae 

 (pi. 67, fig. 4) of several sizes occur on the thick spine sheaths, includ- 

 ing the outer of the two adambulacral spines. In Plazaster there 

 are no adambulacral spine pedicellariae, and none of the spinelets 

 has a fleshy sheath, but a thin one. The absence of pedicellariae 

 attached to one or more adambulacral spines is important in 

 connection with their presence in Labidiaster. Adambulacral spine 

 pedicellariae are found only in the following genera: Asterias, 

 Evasterias, Leptasterias, Stenasterias, Perissasterias, Labidiaster, and 

 sometimes in Ur asterias as a variation. The majority of genera 

 of Asteriidae have no adambulacral spine pedicellariae, although 

 pedunculate straight pedicellariae, attached to adambulacral plates, 

 are of common occurrence. 



The skeleton of Plazaster is different from that of Labidiaster. 

 The very numerous small plates form an extensive irregular net to 

 the tip of ray, without a differentiated carinal and superomarginal 

 system, and the inferomarginal plates alternate with an interpolated 

 secondary ossicle. In Labidiaster the skeleton of the outer half or 

 two-thirds of ray undergoes reduction to widely spaced transverse 

 arches, which in old specimens degenerate to the rudiments of the 

 marginals and a few tiny abactinals. The thick ruffs of giant pedi- 

 cellariae overlie these disappearing or vanished skeletal arches. The 

 rays of Labidiaster like those of Pycnopodia are very flexible. Both 

 animals can catch active prey. 



It is difficult to name the genus to which Plazaster is most closely 

 related and therefore to place it satisfactorily in the system. It 

 differs from the Labidiasterinae in the type of reticulate skeleton, in 

 the inferior status of the marginal plates, and in lacking prominent 

 stoles of pedicellariae on the principal spines. At the same time the 

 ambulacral plates are rather too primitive and uncrowded for the 

 Asteriinae. The genus is in fact very isolated. The weak but 

 complete skeleton, in conjunction with the primitive ambulacrals, 

 suggests the Pedicellasterinae. All known species in this subfamily 

 have five or six rays and lack an adoral carina, the first pair of adam- 



