The right whales were first classified in the genus "Balaena" in 1758 by Carl Linnaeus, who at the time considered all of the right whales (including the bowhead) as a single species. Through the 1800s and 1900s, in fact, 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, and John Edward Gray proposed the genus "Eubalaena" for the right whale in 1864. 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, 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).