Fam. VII. OPISTHOCOMIDAE.–The curious and highly specialized Hoatzin (_Opisthocomus cristatus_) has been the subject of much discussion among systematists, as the outcome of which it is necessary to adopt for it a special Sub-Order OPISTHOCOMI. Buffon classed it with the Curassows, P. L. S. Müller and Gmelin placed it in the Linnean genus _Phasianus_; but Illiger recognised a genus _Opisthocomus_, while Huxley and Garrod fully admitted its claim to higher rank than that of a Family. The habits are to some extent Ralline, and certain points of structure indicate a considerable affinity to the _Cuculi_. The sternum is utterly unlike that of any other species, the anterior portion of the keel being aborted, and the posterior correlated with a flattened area of thick naked skin, on which the bird mainly rests. These modifications are no doubt connected with the extraordinarily large crop, which is supported by the furcula and the fore-part of the breast-bone, being received in a cavity of the pectoral muscles; the whole organ is decidedly muscular, and contains two divisions with a partial constriction between them. The body is long and thin, the bill is strong with basal serrations on the maxilla; bristles surround the gape, and the eye-lids have distinct lashes–a rare fact among birds. The reticulated metatarsi are fairly stout; the toes are long; the hallux being unusually developed and the claws slightly curved. The short rounded wings have ten primaries and nine secondaries. The nearly even tail is elongated, with ten stiff feathers. The plumage in both sexes is olive above with white markings, and dull rufous below; the long loose crest and the tip of the tail are yellowish, and a patch of bare bluish-black skin surrounds the eyes. The tongue is sagittate, the furcula is Y-shaped and ossifies anteriorly with the coracoids, an aftershaft is present, the down of the adults is sparing, while a small amount–of a reddish-brown colour–is observable in the newly-hatched {242}young. The syrinx has one pair of muscles inserted on the distal end of the trachea.