be noted that this species retains throughout life the large slightly concave operculum, with the same relative proportions as in the embryo of zelandicus. The gill filaments are strongly developed and powerfully ciliated, though from their triangular shape, as well as from the lack of an endostyle and of a specialized food groove, it would appear that members of this group never become so completely adapted for ciliary feeding as some of the other style-bearing prosobranchs. Moreover, although the pedal gland in novae-hollandiae is much less well-developed than in either zelandicus or gigas it is yet of relatively considerable size. It may be suggested that this structure was not originally developed in the Vermetidae solely for the production of mucus for ciliary feeding, for which requirement the food tract mucus supply would seem sufficient. Although novae-hollandiae—occurring as stressed by Yonge in a wave-beaten environment—does not resort at all to mucus trap feeding—it is likely that most other species of this group may—like zelandicus—possess to some extent the ability to form mucus traps. Bivonia triqueter is a mucus trap feeder according to Yonge (1932), while on the north American Pacific coast, Spiroglyphus lituellus and Aletes squamigerus—as figured by Johnson and Snook (1935) have the same structure of head, foot, pedal tentacles and food tract as described above. Ricketts and Calvin (1948) briefly state the last-named species feeds by the formation of mucus strings. “Vermetus” maximus and “Vermetus” giganteus are stated by Yonge and Iles (supr. cit.) to possess the more primitive ciliary feeding condition of novae-hollandiae, though their exact relationship to the above species has yet to be ascertained.

It is probable—as suggested by Yonge and Iles (1939)—that the presence of operculum and a powerful ctenidial current tends to impede the employment of mucus strings. In zelandicus, however, mucus feeding appears to be carried out to a minor extent in the presence of a strong current passing into the pallial cavity. Pari passu with the loss of the operculum and the enlargement of the pedal gland, the ctenidium is reduced in size in vermetids, reaching its smallest proportions in gigas, which is probably mainly dependent on pallial respiration. A parallel evolutionary trend seems to be the reduction of the crystalline style. According to Yonge's figure the style is large and stout in novae-hollandiae; in zelandicus it is much more slender, and very delicate and impermanent. Its presence in gigas was first denied by Yonge (1932), but examination of living material (Yonge and Iles, 1939) later revealed a style and a small gastric shield “relatively much smaller than in ‘Vermetus’ novae-hollandiae.” It would be interesting to find whether the style is ever actually lost in mucus trap feeders; the ingestion of predominantly animal material such as small isopods captured by gigas (Boettger, 1930) would greatly diminish the need of a style. Yonge later found small gastropod shells in the stomach of gigas, and it seems well established that the mucus feeders are collectors of zooplankton. The presence or absence of a crystalline style is primarily a physiological character, which is probably not of major taxonomic importance—witness its separate appearance in gastropods in such groups as the prosobranchs and the thecostomatous pteropods.