ASTEROIDEA i8i 



Genus Myoraster nov. 



Diagnosis. Rays 5. Differing from Solaster, Crossaster, and Lophaster in having a 

 specialized internal longitudinal muscle lying between adambulacral and superomarginal 

 plates and extending from end of ray to interbrachial angle, whence it continues to the 

 tip of adjacent ray. Abactinal paxillae resembling those of Lophaster ; marginal plates of 

 So/aster endeca type ; actinal and adambulacral armature not essentially different from 

 that of Lophaster. Type, Lophaster antarcticus Koehler. 



Remarks. The specialized actinolateral muscle is the most characteristic feature of 

 this genus, which otherwise resembles a combination of Solaster and Lophaster. The 

 marginal plates, for example, are like those of typical Solaster, since the inferomarginal 

 paxillae are compressed, somewhat fan-shaped, while the superomarginal paxillae are only 

 a trifle larger than the abactinal paxillae and usually stand above the interspaces between 

 the inferomarginals, as in Solaster endeca. In typical Lophaster the conspicuously enlarged 

 superomarginal paxilla stands above the inferomarginal and the combined plates form a 

 single strong dorsoventral calcareous ridge, very apparent when viewed from the coelom. 



It is of course not possible to make a definitive evaluation of such a character as the 

 actinolateral, or marginal, muscle which appears thus in an isolated species. The slight 

 rudiment of it, as a faint narrow band, can be detected in some specimens of Solaster 

 endeca, where it can hardly be of functional importance. I can find no trace of it in 

 Lophaster. An actinal muscle layer, of which at least a part is homologous with this 

 marginal muscle, occurs in Cuenotaster, a genus very distinct from all other Solasterids. 



An interesting feature of the type species is a specialized spinelet terminating in 3 

 lobes, rather than 3 sharp prongs. The variations have been fully figured by Koehler 

 (1920, pi. 67, fig. 3). This special spinelet occurs in addition to the normal spinelets with 

 3 sharp terminal prongs, and has not been described in any other Solasterid. 



It is possible that Solaster incertus Koehler (1917, p. 40, pi. 5, figs. 2, 3 ; pi. 9, figs. 21, 

 22) belongs to this genus. The type and only known specimen from Kerguelen is too 

 small (R 15 mm.) to indicate adult characters. 



Lophaster tenuis Koehler, based upon a single small specimen (R 21-22 mm.) from 

 64° 32' S, 97° 20' E, no fathoms, has very special spine characters which separate it 

 from afitarctkus. It may belong to Myoraster (Koehler, 1920, p. 151, pi. 32, figs. 1-7; 



pi. 68, fig. 5). 



Myoraster antarcticus (Koehler) 



Lophaster antarcticus Koehler, 1912, p. 46, pi. 3, figs. 4, 5.— 1920, p. 144, pi. 32, figs. 8-11; pi. 33, 

 figs. 3, 4; pi. 67, figs. 1-5; pi. 68, figs. I, 2.— 1923, p. 75. 



St. 148. Off Cape Saunders, South Georgia, 132-148 m., grey mud, stones, 4 specimens. 



St. 175. Bransfield Strait, South Shetlands, 63° 17' 20" S, 59^ 48' 15" W, 200 m., mud, stones, 

 gravel, i very small specimen. 



St. 181. SchollaertChannel, Palmer Archipelago, 64° 2o'S,63°oi'W, 160-335 m., mud, 5 specimens. 



St. 182. Schollaert Channel, Palmer Archipelago, 278-500 m., i specimen. 



St. 371. One mile east of Montagu Island, South Sandwich Islands, 99-161 m., i specimen. 



St. 599. West coast of Adelaide Island, 67'' 08' S, 69° 06J' W, 203 m., 3 specimens. 



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