EASTERN TROPICAL PACIFIC ASTEIIOIDEA. 99 



Key to the species of Pliolidaster. 



Superomarginals and most of the disk-plates concealed by flat, skin-covered scales sqtuimatv3. 



Superoniarginals and primary disk-scales largely bare distinctus. 



Myxoderma. 



Fisher, 1905. Bull. V. S. Bur. Fish., 24, p. 316, as a subgenus of Zoroaster. 

 Type-species: M. sacculatus Fisher, 190.5. Loc. cit. 



This small group of sea-stars occurring off the coast of the western United 

 States and Lower California in 284-916 fms. is well-circumscribed and Fisher 

 has wisely (I9I9. Ann. Mag. Nat. Hist., ser. 9, 3, p. 392) raised it to full generic 

 rank. In the same paper he has described a subspecies of each of the two com- 

 ponent species but I doubt whether sufficient material has yet been secured to 

 warrant recognition of subspecies. In any case, however, he has set forth so 

 well the characters of these supposed subspecies in his key, that we need not dis- 

 cuss them here. There are certainly two valid species. 



Key to the species of Myxoderma. 



Spines and spinelets more or less conspicuously saeculate; big pedicellariae in ambulacral furrow or on 

 adambulacral spines; papulae in adults 3-5 in an area on base of rays abactinally . . . . sacculatum. 



Spines and spinelets little or not at all saeculate; no big pedicellariae in furrow or on adambulacral spines; 

 papulae 1 or 2 in an area on base of rays abactinally platyacanthum. 



Bythiolophus. 



Fisher, 1916. Proc. Biol. Soc. Wash., 29, p. 31, 1919. Bull. 100 U. S. N. M., p. 484, pi. 135, fig. 1-lc, 



139, fig. 1, 2. 

 Type-species: B. acanlhinus Fisher, 1916. Loc. cit. 



This monotypic genus is based on a single sea-star taken by the Albatross 

 in Buton Strait, Celebes, in 559 fms. It is unquestionably a weU-marked and 

 noteworthy type. In the diagnosis as pubhshed in 1919, the presence of sub- 

 ambulacral plates is mentioned; Dr. Fisher assures me this is a slip for super- 

 amhulacral. In the key to genera of Zoroasteridae (1919, p. 471) Bythiolophus 

 is said to have no adradial plates and in the figures 1 and Ic, pi. 135, as well as 

 in the explanation of the plate, the series of plates adjoining the carinals on 

 each side is called the superomarginal. But in the diagnosis of the genus (p. 484) 

 and in the diagnosis and description of the species (p. 485) this series is consid- 

 ered as adradial and the prominence of the adradial plates is emphasized. Dr. 

 Fisher writes me that the key was prepared much later than the text and is 



