This family, like Phaethontidae, Sulidae, and Fregatidae, consists of a single living genus, which is distributed throughout the warm and temperate regions of the earth. There are 17 cervical vertebrae. The ambiens and all other of the characteristic thigh-muscles are absent, except the femoro-caudal. No syringial muscles.
The Fauna Of British India, Including Ceylon And Burma-birds
Blanford, William Thomas, ed. The Fauna of British India: Including Ceylon and Burma. Vol. 4. 1898.
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