Lagenorhynchus

Lagenorhynchus is a genus of oceanic dolphins in the infraorder Cetacea, presently containing six extant species. However, there is consistent molecular evidence that the genus is polyphyletic and several of the species are likely to be moved to other genera. In addition, the extinct species Lagenorhynchus harmatuki is also classified in this genus.

Etymology
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 popularly called bottlenose dolphins belong in the genus Tursiops.

Taxonomy
There is compelling phylogenetic molecular evidence that the genus Lagenorhynchus is polyphyletic, in that it currently contains several species that are not closely related. found that white-beaked and Atlantic white-sided dolphins are phylogenetically isolated within the Delphinidae, where they are believed to be rather basal members of the family, along with the orca (subfamily Orcininae).

The remaining four species in the genus Lagenorhynchus—the Pacific white-sided dolphin, Peale's dolphin, hourglass dolphin and the dusky dolphin—are consistently placed within the Lissodelphininae subfamily, in studies of molecular phylogeny, together with the right whale dolphin and the four species of the genus Cephalorhynchus (including Hector's dolphin). Some authors have suggested these four species be placed in the resurrected genus Sagmatias. However, other molecular studies place the hourglass and Peale's dolphins, phylogenetically, within the genus Cephalorhynchus and do not agree with inclusion in a new genus (together with Pacific white-sided dolphin and dusky dolphin).

This phylogeny is supported by acoustic and morphological data; both the hourglass and Peale's dolphins share, with the other species of Cephalorhynchus, a distinctive type of echolocation signal known as a narrow-band/high-frequency signal. This signal is also used by porpoises (Phocoenidae) and the pygmy sperm whales (Kogiidae), but is not found among other dolphins. According to, Peale's dolphin, and the other Cephalorhynchus species, are the only dolphins that do not "whistle"; presumably, this would be the case for hourglass dolphins, as well. Peale's dolphin also shares with several Cephalorhynchus species the possession of a distinct white marking behind the pectoral (“armpit”) fin.

The melon-headed whale was first classified as member of the genus Lagenorhynchus, but was later moved to its own genus, Peponocephala.