ASTEROIDEA OF NORTH PACIFIC AND ADJACENT WATERS — FISHER 45 



of the first puir of ambulacral ossicles and the body wall. This forms in effect an 

 inner accessory actinostomial ring and is not found at all in true Zoroaster. 



Zoroaster evermanni does not possess these specialized stays, nor other superam- 

 bulacral plates, and its adradial plates are more nearly like those of Zoroaster than 

 those of Myxoderma. 



AI. platyacanthum is a very distinct form and the membrane is not so thick and 

 pulpy as in sacculatum, nor is there evidence that it was slimy in life. The name 

 Myxoderma is therefore in part a misnomer, although highly descriptive of the type 

 species, sacculatum. 



KEY TO THE KNOWN SPECIES AND SUBSPECIES OP MYXODERMA 



1 . Size large (R, 200 mm.), the plates and spines invested in a thick pulpy membrane, slimy in 



life; abactinal papular areas large, the dorsal skeleton being open and rather irregularly 



reticulate at base of ray; abactinal plates without accessory spinelets to any extent; actino- 



lateral spines pointed, tapered, sometimes somewhat flattened but never spatulate; terminal 



plate large, ovoid, especially conspicuous in young specimens; rays alternate distally. 



b l . Rays thicker and skeleton stouter; third or lowest series of actinolateral plates extending 



five-eighths the length of the ray; abactinal spines robust and stubby; abactinal pedi- 



cellariae smaller and less numerous. Bering Sea to central California.. sacculatum Fisher. 



b 1 . Rays longer and slenderer, the third or lowest actinolateral series of plates extending along 



only the proximal third of ray; spines rather slenderer, and abactinal pediceUariae larger 



and more numerous. California south of Point Conception ectenes Fisher. 



2 . Size medium, the plates and spines not especially sacculate or slimy — about as in Zoroaster; 



abactinal papular areas with one or two papulae which do not occupy all the area; adradial 

 plates small, more or less overlapped by the carinals and superomarginals; abactinal plates 

 with numerous accessory miliary spinelets; actinolateral spines broad, flat, and more or 

 less truncate; terminal plate small; rays not alternate distally. 

 6 1 . Rays slenderer and longer, disk smaller; plates, especially the carinal and marginal, not 

 broader than long; superomarginals not noticeably prominent; spines slenderer; pedi- 

 ceUariae larger. Southern California (south of Point Conception) and Lower Cali- 

 fornia platyacanth um (Clark) . 



b 1 . Rays broader and shorter, disk slightly larger; plates, especially the carinal and marginal, 

 broader than long; superomarginals noticeably prominent, at least distally; spines more 

 robust; pediceUariae shorter. Oregon to Central California rhomaleum Fisher 



MYXODERMA SACCULATUM Fisher 



Plate 14, Figure 5; Plate 15, Figures 1, la-lc; Plate 20, Figure 2; Plate 21, Figures 2, 3; Plate 22, 



Figures 2, 3; Plate 25, Figure 4 



Zoroaster (Myxoderma) sacculatus Fisher, Bull. Bureau Fisheries, 1904, vol. 24, June 10, 1905, 



p. 316. 



Zoroaster evermanni Clark (not Fisher), Bull. Amer. Mus. Nat. Hist., vol. 32, art. S, July 



9, 1913, p. 198. 



Myxoderma sacculatum Fisher, Ann. and Mag. Nat. Hist., ser. 9, vol. 3, 1919, p. 392. — 



Clark, Mem. Mus. Comp. Zool., vol. 39, No. 3, p. 99; BuU. Amer. Mus. Nat. Hist., 



art. 6, 1923, p. 152. 



Diagnosis. — Rays five. R 200 mm., r 17 mm., R= 11.7 r; breadth of ray at base, 

 22 mm. Disk convex, fairly high; rays subcylindrical, stout but slender, the base 

 being slightly inflated, tapering thence evenly to an attenuate extremity capped by a 

 swollen conspicuous terminal plate; rays not carinate except on outer portion, the 

 dorsal surface proximally being evenly arched. Abactinal skeleton very open, with 

 large papular areas containing several papulae; all plates with a short stout spine 

 64406—28 4 



