6 
THE VOYAGE OF H.M.S. CHALLENGER. 
Against such a tendency it is necessary to protest and to revert to a less rigid con- 
ception of what constitutes a genus, and though one does attach considerable systematic 
importance to the shell, this must not be exaggerated so as to lead to the erection of a 
separate genus for species which differ in some minute feature in the shell, but agree 
with one another in the rest of their characters. In addition to several differences of 
some importance in regard to the shells, it will be necessary to justify the establishment 
of a genus by at least one characteristic difference either in the soft structure of the 
animal, or in certain hard parts like the operculum or the buccal armature, which are 
generally of real importance in generic and specific diagnoses. 
From this it follows that a large number of the generic titles discussed below must 
either .be abandoned or regarded as synonyms. Some of them again may be considered 
as designating generic subdivisions, though these are not in any way indispensable in so 
comparatively small a group as the Thecosomata. 
I shall append, from the literature of the group, an alphabetical list of the various 
generic names given to living Thecosomata : — 
Agadina, Gould. 
Archonta, Montfort. 
Balantium , Leach. 
Campylonaus, Gray. 
Cavolinia, Abildgaard. 
Cleodora, Peron and Lesueur. 
Clio, Linne. 
Corolla, Dali. 
Creseis, Rang. 
Cuvieria, Rang, non Peron. 
Cuvier ina, Boas. 
Cymbulia, Peron and Lesueur. 
Diacria, Gray. 
Embolus, Jeffreys. 
Euromus, H. and A. Adams. 
Gleba, Forsk&l. 
Ileliconoides, d’Orbigny. 
Helicophora, Gray. 
Heterofusus, Fleming. 
Hyalsea, Lamarck. 
Hyalocylis, Fol. 
Limacina, Cuvier. 
Orbignyia, A. Adams. 
Peracle, Forbes. 
Pleuropus, Eschscholtz. 
Protomedea, 0. G. Costa. 
Rheda, Humphreys. 
Scsea, Philippi. 
Spiratella, de Blainville. 
Spirialis, Eydoux and Souleyet. 
Styliola, Lesueur. 
Tiedemannia, delle Chiaje. 
Tricla, Oken, non Retzius. 
Triptera, Auctorum, non Quoy and 
Gaimard. 
Of these thirty-four titles 
I. Two ought to be provisionally set aside as doubtful, for reasons which I shall after- 
wards submit : — 
Agadina, Gould. 
Triptera, Quoy. and Gaimard. 
