dú / jiǎo / jīng / tóu / shàng / de / cháng / yá / qí / shí / shì / yì / kē / tū / chū / de / yá / chǐ独角鲸头上的长牙其实是一颗突出的牙齿。The long tusk on a narwhal's head is actually a protruding tooth.