65 



cause that these "species" are now very vaguely known, and will r(;main so in the literature on 

 the Gymnosomata, without much possibility that other naturalists may recognize them with certainty. 



Though the studies of Cuvier, Eschricht, df, Blainville, van Beneden, Souleyet, 

 Gegenbaur, Troschel, Muller, Fol, Krohn and Wagner — treating of whole groups or of 

 a single species only — have decidedly their merits, it was not until Boas and Pelsexeer took 

 up their tasks, that the systematic classification of the Gymnosomata became clearly established. 

 Many of the species formerly described, ought to be obliterated as real species, as their 

 characterization is insufficient, only a few should remain. On the other hand, the number ot 

 distinct species was importantly increased by the two last-mentioned zoologists. The result of their 

 monographs was, that twent)-one species were then really known. These species belonged to 

 five families and seven genera (Pelseneer). 



It may be supposed, however, that we know only a small number of the species, reallj- 

 existino- in our seas. So, from the Mediterranean some peculiar larvae were formerly described 

 by Gegenbaur'), which since that time no naturalist has found again, at least not in the adult state, 

 but which belong to a genus Thliptodon, established by Boas "). Many of the species, badly 

 described b)' previous authors, may nevertheless represent curious t)pes, about the characters 

 of which we unfortunately do not possess sufficient information. According to the preliminary 

 notes of Meisenheimer, there has already been found in the material of the Valdivia Expedition 

 a new family, Pteroceanidae ^), and one new genus Schizobrachhiin ^). 



And finally the results of the Siboga Expedition show that explorations in a rather 

 limited area, as the eastern part of the Indian Archipelago, may procure very sufficient results. 

 For six genera, one of which is new, are represented in the collection, while the number of 

 species amounts to eight. I""ive of them had not been described before. 



Family I. PNEUMONODERM.vnOAE. 



1840. PneiiinodcDuidae (part.) Gray, Synopsis of the contents of the British Museum, p. 86. 

 1846. Fneiaiiodennidae Agassiz, Nomenclator zoologicus, Index, p. 299. 



1853. Fneumodcnnidae (part.) d'Orbigny in Ramon de la Sagra, Histoire physique, politique 

 et naturelle de I'ile de Cuba, Mollusques, vol. II, p. 70. 



1855. Clioldea (part.) Gegenbaur, Untersuchungen iiber Pteropoden und Heteropoden, p. 212. 



1856. Cliidae (part.) Woodward, A Manual of the Mollusca, p. 20S. 



1858. Pneinnodermonidae (part.) H. and A. Adams, The Genera of recent Mollusca, vol. I, p. 62. 



1862. Pneumodennidae Bronn, Die Klassen und Ordnungen des Thierreichs, Bd. Ill, p. 645. 



1863. Pneumodermacea Troschel, Das Gebiss der Schnecken, Bd. I, p. 56. 



1 87 1. Fnetimodcrinatidae Dall, Description of si.xty new forms of Molluscs from the West 



coast of North America, Americ. Journ. Conch., vol. VI, p. 139. 

 1887. Pneuinonodi-rmatidae Pelseneer, Challenger Report, LYIII, p. 11. 



1) Untersuchungen iiber Pteropoden und Heteropoden, p. 95 — 97, pi. V, fig. 14- 



2) Spolia atlantic.n, p. 174. 



3) Zool. Anz., Bd. XXVI, p. 92 — gg. 



4) Ibid., p. 410 — 412. 

 SIROGA-EXPEDITIE LU. 



