Lagenorhynchus is a genus in the order Cetacea, traditionally containing six species:

• white-beaked dolphin, Lagenorhynchus albirostris
• Atlantic white-sided dolphin, Lagenorhynchus acutus
• Pacific white-sided dolphin, Lagenorhynchus obliquidens
• dusky dolphin, Lagenorhynchus obscurus
• Peale's dolphin, Lagenorhynchus australis
• hourglass dolphin, Lagenorhynchus cruciger

The name Lagenorhynchus derives from the Greek lagenos meaning bottle and rhynchus meaning "beak". Indeed the "bottle-nose" is a characteristic of this genus. However the dolphins that have attained the common name bottlenose dolphin belong in the genus Tursiops. The melon-headed whale was once classified in this genus but was later removed to its own Peponocephala genus.

Recent analysis of the mitochondrial cytochrome b gene indicates that the genus Lagenorhynchus, as traditionally conceived, is not a natural (monophyletic) group. LeDuc et al. (1999) found that the white-beaked and Atlantic white-sided dolphins were phylogenetically isolated within the Delphinidae, whereas the remaining four species were members of the Lissodelphinae, a predominantly Pacific clade of dolphins also including the right whale dolphins and the Cephalorhynchus dolphins. These findings are somewhat problematic taxonomically, since the white-beaked dolphin is the type species of the genus Lagenorhynchus; if the other species are not closely related to the white-beaked dolphin, then they must be removed from the genus. Accordingly, LeDuc et al. suggested that the Atlantic white-sided dolphin be placed within its own genus, Leucopleurus, and that the remaining species would need taxonomic revision as well. Ledouc proposed Sagmatias as the new genus for the Pacific white-sided dolphin, Peale's dolphin, hourglass dolphin and dusky dolphin.^[1]

May-Collado & Agnarsson (2006) actually recovered the hourglass and Peale's dolphins as nested phylogenetically among the species of Cephalorhynchus, and they suggest that these two species be transferred to that genus. There is some acoustic and morphological support for this arrangement, at least with respect to Peale's dolphin. According to Schevill & Watkins (1971), Peale's dolphin and the Cephalorhynchus species are the only dolphins that do not whistle (no acoustic data are available for the Hourglass dolphin). Peale's dolphin also shares with several Cephalorhynchus species the possession of a distinct white "armpit" marking behind the pectoral fin.

According to May-Collado & Agnarsson's analysis, the remaining two species, the dusky and Pacific white-sided dolphins are closely related to each other and form the sister group to the (expanded) genus Cephalorhynchus. If this placement is accurate, a new genus name will need to be coined to accommodate these two species.

For further details, please see the articles on each species.

References

1. LeDuc, R.G., Perrin, W.F., Dizon, A.E. (1999). Phylogenetic relationships among the delphinid cetaceans based on full cytochrome b sequences. Marine Mammal Science 15, 619–648.
2. May-Collado, L., Agnarsson, I. (2006). Cytochrome b and Bayesian inference of whale phylogeny. Molecular Phylogenetics and Evolution 38, 344-354.
3. Schevill, W.E., Watkins, W.A. (1971). Pulsed sounds of the porpoise Lagenorhynchus australis. Breviora 366, 1–10.