10 



THE VOYAGE OF H.M.S. CHALLENGER. 



III. Because they have been insufficiently characterised, never again gathered, and 

 the type specimens have not been preserved ; they are therefore too doubtful to be 

 introduced into systematic nomenclature : — 



1. Pelagia, Quoy and Gaimard. 1 



2. Cymodocea, d'Orbigny. 2 



3. Homoderma, van Beneden. 3 



Among the genera hitherto described I only admit six and the new genus 

 Notobranchsea (these six genera are also those that were accepted by Boas, 4 before the 

 description of Notobranchsea). They are the following : — 



1. Dexiobranchsea, Boas. 



2. Spongiobranchsea, d'Orbigny. 



3. Pneumonoderma, Cuvier. 



4. Clionopsis, Troschel. 



5. Clione, Pallas. 



6. Halopsyche, Bronn. 



These six genera, and the new one Notobranchsea, are well known, well charac- 

 terised and distinct. The following table gives their principal distinctive characters :— 



Key to the Genera. 



I. A specialised branchial apparatus. 



1. A lateral gill. 



A. No posterior gill, . 



B. A posterior gill. 



a. A circular posterior gill, 



b. A tetraradiate posterior gill, 



2. No lateral gill. 



A. No buccal appendages 



B. Buccal appendages, 



II. No specialised branchial apparatus. 



1. Body lengthened, posteriorly pointed, 



2. Body ovoid, posteriorly rounded, 



Dexiobranchsea. 



Spongiobranchxa. 

 Pneumonoderma. 



Clionopsis. 

 Notobranchxa. 



Clione. 

 Halopsyche. 



How must we subdivide these seven genera of the Gymnosomatous group \ 

 Fischer, who considers the Pteropoda to be a class of the same value as the Gastro- 

 poda (although the organisation of the Pteropoda shows, in the clearest manner, that they 

 only form a subdivision of the Euthyneurous Gastropoda), distinguishes among the 

 Gymnosomata two suborders :* — Scleroderma, containing only the genus Halopsyche, 

 and Malacoderma, embracing all the other genera. But the character on which this 

 subdivision is based is very insufficient, because the teguments of Halopsyche are not 



1 Voyage de decouvertes de 1' Astrolabe, Zoologie, t. ii. p. 192= Pteropelagia, Bronn. 



2 Voyage dans l'Amerique meridiqnale, t. v. p. 133 = Pterocymodocea, Bronn. 



3 Exercices zootomiques, p 54, pi. iii. figs. 4, 5. 



4 Vorlaufige Mittheilung iiber einige Gymnosomen Pteropoden, Zool. An;:eiger, p. 690, 1885. 



5 Manuel de Conchyliologie, p. 422. 



