Source : Webster's Revised Unabridged Dictionary (1913)

Sheathbill \Sheath"bill`\, n. (Zo["o]l.)
   Either one of two species of birds composing the genus
   {Chionis}, and family {Chionid[ae]}, 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.