What does "sheathbill" mean?

Sheathbill: Either one of two species of birds composing the genus Chionis, and family Chionidæ, native of the islands of the Antarctic.seas. Note: They are related to the gulls and the plovers, but more nearly to the latter. The base of the bill is covered with a saddle-shaped horny sheath, and the toes are only slightly webbed. The plumage of both species is white.