Rorquals are the largest group of baleen whales, with nine species in two genera. They include the largest animal that has ever lived, the blue whale. They take their name from a Norwegian word meaning "furrow whale": all members of the family have a series of longitudinal folds of skin running from below the mouth back to the navel (except the sei whale, which has shorter grooves). They allow the mouth to expand immensely when feeding. All rorquals have these unique folds. The pygmy right whale shares several characteristics with the right whales, with the exception of having a dorsal fin. Also, pygmy right whales' heads are no more than one-fourth the size of their bodies, whereas the right whales' heads are about one-third the size of their bodies. The pygmy right whale is the only extant member of its family. Eschrichtiidae only has one living member: the gray whale. It is the only benthic feeding baleen whale, filtering small organisms from the mud of shallow seas. They also have a gestation period of over a year, which is unusual for baleen whales. The toothed whales (parvorder Odontoceti), as the name suggests, are characterized by having teeth (rather than baleen). Toothed whales are active hunters, feeding on fish, squid, and in some cases other marine mammals. Oceanic dolphins are the members of the family Delphinidae. As the name implies, they tend to be found in the open seas, unlike the river dolphins, although a few species such as the Irrawaddy dolphin are coastal or riverine. The Delphinidae are characterized by having distinct beaks (unlike the Phocoenidae), two or more fused cervical vertebrae and 20 or more pairs of teeth in their upper jaws. None is more than 4 m long. The Monodontidae lack dorsal fins, which have been replaced by tough, fibrous ridges just behind the midpoints of their bodies and are probably an adaptation to swimming under ice, as both do in their Arctic habitat. The flippers are small, rounded and tend to curl up at the ends in adulthood. All, or almost all, the cervical vertebrae are unfused, allowing their heads to be turned independently of their bodies. The dwarf and pygmy sperm whales resemble sperm whales, but are far smaller. They have blunt, squarish heads with narrow, underslung jaws; the flippers are set far forward, close to the head and their dorsal fins are set far back down the body.