244 THE VOYAGE OF H.M.S. CHALLENGER. 



that the form should be referred to Astropecten, if the figure is correct. This view appears 

 to me altogether untenable, unless the description is also wrong. 



There are several starfishes in the Museum at Stockholm, which are referred to this 

 species, and in my opinion correctly, so far as my knowledge, limited to the description of 

 Thomson's type, justifies such an expression. I have not, however, examined the specimens 

 with reference to microscopic details, and therefore confine myself to the simple statement 

 of their existence. The examples in question, which were collected by the " Ingegerd " 

 and "Gladan" Expedition in 1871, were dredged in Baffin's Bay, lat. 67° 26' N., long. 

 58° 29' W., at a depth of 692 fathoms, and off Omenak, on the west coast of Greenland, 

 in 122 fathoms. 



Subfamily Luidiin^e, Sladen, 1887. 



Genus Luidia, Forbes. 



Luidia, Forbes, Mem. "Wem. Soc., 1839, vol. viii. p. 123. 



Hemicnemis, Miiller and Troschel, Monatsber. d. k. Akad. d. Wiss. Berlin, 1840 (April), p. 105. 



Petalaster, Gray, Ann. and Mag. Nat. Hist., 1840 (November), vol. vi. p. 183. 



This well-known and sharply defined genus constitutes a very distinct type, the char- 

 acters of which are remarkably constant and subject to comparatively slight modification, 

 as exhibited in the range of species at present known. 



Two points in the morphological structure of Luidia, which are highly significant from 

 a phylogenetic point of view, may here be referred to in justification of the course I have 

 taken in placing the genus in a distinct subfamily. The first is the correspondence of the 

 infero-marginal and adambulacral plates, and the second is the presence of a small inter- 

 mediate plate between each infero-marginal and adambulacral plate. The correspondence 

 of the infero-marginal and adambulacral plates has already been noticed by Alex. Agassiz l 

 and Viguier ; 2 but the intermediate plate, notwithstanding its importance from a systematic 

 point of view, has strangely hitherto been overlooked by all observers : in fact, the asser- 

 tion of its presence is in direct opposition to the statements of other writers on the group. 

 Thus Viguier, 2 who has made a careful study of the details of the Asterid skeleton, states 

 that in Luidia the marginal and adambulacral plates alone constitute the actinal skeleton 

 of the rays, and that it is only in the interradial angles that intermediate plates— smaller 

 and less numerous than in Astropecten — are intercalated between the two series. The same 

 opinion is held by Perrier, 3 who, in his recent work on the Asteroidea of the " Blake " 

 Expedition, regards the contiguity of the marginal and adambulacral plates as a character 

 diagnostic of the family Astropectinidae, the genus Luidia being included in this category. 

 In the face of these statements I should have hesitated in according a special import- 



1 North American Starfishes, Mem. Mus. Comp. Zool., Harvard, vol. v. No. 1, 1877, p. 117. 



2 Archives de Zool. exper., 1878, t. vii. p. 228. 



3 Nouv. Archives Mus. Hist. Nat, 2e Sene, 1884, t. vi. p. 266. 



