Balaena is a genus of cetacean (whale) in the family Balaenidae. Balaena is considered a monotypic species, as it has only a single extant species, the bowhead whale (B . mysticetus). It was first described in 1758 by  Linnaeus, who at the time considered all of the right whales (and the bowhead) as a single species. Historically, both the family Balaenidae and genus Balaena were known by the common name, "right whales", however Balaena are now known as bowhead whales. Throughout history, the family Balaenidae has been the subject of great taxonometric debate. Authorities have repeatedly recategorized the three populations of right whale plus the bowhead whale, as one, two, three or four species, either in a single genus or in two separate genera. In the early whaling days, they were all thought to be a single species, Balaena mysticetus. Eventually, it was recognized that bowheads and right whales were in fact different. Later, morphological factors such as differences in the skull shape of northern and southern right whales indicated at least two species of right whale—one in the Northern Hemisphere, the other in the Southern Ocean. As recently as 1998, Dale Rice, in his comprehensive and otherwise authoritative classification, Marine mammals of the world: systematics and distribution, listed just two species: Balaena glacialis (the right whales) and Balaena mysticetus (the bowheads).