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



definitive results. The first, and a very commendable, attempt to bring light into this 

 chaos, was made by Boas, 1 and in the following systematic account of the genus 

 Pneumonoderma I have availed myself of the results he has obtained ; this account is 

 thus more perfect, although it cannot be considered as conclusive. 



I have found in the literature of the Pteropoda eleven different specific names 

 attributed to Pneumonoderma. 



Among these eleven names there is one that belongs to a Dexiobranchsea — namely, 

 Pneumodermon ciliatum, Gegenbaur, and among the ten others, there are — 



First, names which are synonyms, as will be shown further on : — 



1. Pneumodermon audebardi, Eang, 2 = Pneumonoderma violaceum, d'Orbigny. 



2. AZgle cucullata, Oken, 3 = Pneumonoderma per oni, Lamarck. 



3. Pneumodermis atlanticum, Oken, 4 = Pneumonoderma violaceum, d'Orbigny. 



4. Pneumodermon macrocotylum, Boas, s = Pneumonoderma mediterraneum, 



van Beneden. 



Secondly, several names applied to species of which the characterisation is quite 

 insufficient, and of which the type specimens have been lost. One is therefore obliged 

 to eliminate them from the actual systematic nomenclature of Pneumonoderma : — 



1. Pneumodermon pellucidum, Quoy and Gaimard. 



2. Pneumodermon ruber, Quoy and Gaimard. 7 

 I shall, however, refer again to these two species. 



The obscurity which surrounds the nomenclature of the genus Pneumonoderma 

 results from the difficulty of finding distinctive characters between the different species, 

 so that the descriptions which have been given of some forms might have been applied 

 to others, which are nevertheless distinct. It may therefore be said that the real cause 

 of this confusion is the fact that the zoologists who have undertaken the systematic 

 classification of the Mollusca possess but little knowledge of the organisation of the 

 animals that they describe, and therefore cannot appreciate which organs maintain 

 constant characters throughout the whole genus, and which have characters that vary 

 from one species to another. To convince oneself of this truth, one must compare the 

 descriptions of Quoy and Gaimard, of Rang, and even of d'Orbigny and Dall, with those 

 of Boas, and he will then understand the accuracy of this observation. 



The first well-known species of the present genus was Pneumonoderma peroni, 

 Lamarck, from the Indian Ocean. 



1 Spolia atlantica, pp. 152-154. 



2 Histoire naturelle des Mollusques Pteropodes, pi. x. fig. 13. 



3 Lehrbuch der Zoologie, Bd. i. p. 327 (err. typ., 273). 



4 Loc, cit., p. 327. 



6 Spolia atlantica, p. 152. 



6 Voyage de decouvertes de l'Astrolabe, Zoologie, t. ii. p. 390, pi. xxviii. fig. 29. 



7 Loc. cit., t. ii. p. 389, pi. xxviii. figs. 19-24. 



