Adpos=a ravenous sea-bird, in classical Greek. Akin to Old Norse liri=a Tern.
Bill strong, and always more than twice and usually about three times as long as it is deep. Wings long, exceeding tail. First primary longest. Some species in summer have the head darker than the neck.— In winter; Head and neck are white, or streaked with brown. For purposes of distinction these birds are here given in three groups, viz.—
A. Head with hood darker than neck in summer (227-231).
B. Head without hood and tail white (232-235).
C. Head without hood and tail with subterminal black band. (Cosmopolitan with exception of Polynesia and the Central Pacific.)
A. Head with defined hood darker than the neck in summer. Hood black. Mantle grey.