It has a long and slightly-upturned beak with 31—36 conical teeth on either jaw. Its dorsal fin is low and triangular in shape and resembles a light-colored flag when the dolphin swims just below the surface of the murky Yangtze River , hence the name "white-flag" dolphin. It has smaller eyes compared to oceanic dolphins. Because of its poor vision, the baiji relies primarily on sonar for navigation. Sound emission is focused and highly directed by the shape of the skull and melon. This had been reduced by several hundred kilometres both upstream and downstream, and was limited to the main channel of the Yangtze, principally the middle reaches between the two large tributary lakes, Dongting and Poyang.