54 THE VOYAGE OF H.M.S. CHALLENGES. 



Pysche globulosa, and which he considered to be one without a distinct head. But since 

 the name Pysche had already been applied by Linne to a genus of butterflies, it had to be 

 eliminated from the nomenclature of the Pteropoda, and Bronn, therefore, substituted 

 the name Halopsyche, which has been adopted by Verrill, 1 Lankester 2 and Boas. 3 



In 1827 Eang formed another new genus, Euribia, for a Pteropod that he thought 

 to be testaceous (a supposition since proved to be incorrect) ; but here again the name 

 Eurybia had been already used by Hiibner (1806), and it could not be retained for a 

 Pteropod, and Bronn therefore changed it to Theceurybia. 



But, on the other hand, Souleyet 4 showed that the two genera "Psyche " and "Euribia" 

 of Rang are not distinct, and the name Halopsyche must therefore be retained for the 

 whole group. 



Eang, 5 Cuvier, 6 Gray, 7 Owen, 8 Woodward, 9 Adams, 10 Bronn u and Lankester, 12 have 

 all considered these animals as Thecosomatous Pteropods ; Macdonald 13 and Boas 14 alone 

 maintaining that they are Gymnosomata. This last opinion is the true one. 



Halopsyche indeed does not possess any character of the Thecosomata ; on the 

 contrary, it shows all the characters common to' the Gymnosomata ; it possesses neither 

 mantle skirt, shell, nor pallia! cavity ; it has a distinct head, bearing two pairs of 

 tentacles (the posterior with rudiments of eyes) ; the foot and fins are separated ; there 

 are buccal appendages and a jaw of which the two halves are united in the median line ; 

 there are no stomachal horny plates ; the anus is on the right side (and not on the left, 

 as stated by Huxley 15 ) ; the cerebral ganglia are united above the oesophagus ; and 

 finally, the larvae resemble those of all the Gymnosomata (PI. III. fig. 9). 



Halopsyche therefore has no relation to the Thecosomata. If its radula seem to 

 resemble that of the Thecosomata in the number of the teeth, it differs from it by the 

 conformation of the teeth, as may be seen below. 



Halopsyche can no longer be regarded, as some zoologists believe, as a transitional 



1 Notice of Recent Additions to the Marine Invertebrata of the North- East Coast of America, Proc. U.S. Nat. Mus., 

 vol. Hi. p. 393. 



2 Mollusca, Encyclopaedia Britannica, ed. 9, vol. xvi. p. 666. 



3 Spolia atlantica, p. 171. 



4 Voyage de la Bonite, Zoologie, t. ii. p. 252. 



6 Description de deux genres nouveaux appartenant a la classe des Pteropodes, Ann. d. Sci. Nat., s6r. 1, t. xii. 

 p. 328. 



6 Le Regne animal. 



7 Catalogue of the Mollusca in the Collection of the British Museum, pt. ii., Pteropoda, p. 27. 



8 Mollusca, Encyclopaedia Britannica, ed. 8, vol. xv. p. 381. 



9 A Manual of the Mollusca. 



10 The Genera of Recent Mollusca, vol. i. p. 56, 57. 



11 Die Klassen und Ordnungen des Thierreichs, Bd. iii. pp. 645. 



12 Mollusca, Encyclopedia Britannica, ed. 9, vol. xvi. p. 666. 



13 On the Anatomy of Eurybia gaudichaudi, Trans. Linn. Soc. Lond., vol. xxii. p. 248. 



14 Spolia atlantica, p. 171. 



15 On the Morphology of the Cephalous Mollusca, Phil. Trans., p. 41, 1853. 



