OPISTHOCOMIDAE 24 1 



this formation, but from the Upper Miocene of Oeningen and the 

 Pliocene of Attica, in the latter of which Gallus accompanies it. 

 Mekagris has been discovered in the Miocene of Colorado, and 

 the Post -pliocene of New Jersey; Gallus in the Pliocene of 

 France, Palaeotetrix and Fedioecetes in that of Oregon; and, 

 iinally, bones of Lagopus have been brought to light in the 

 French Plistocene, and those of Tetrao urogallus at Kent's Hole 

 near Torquay and in the caves of Teesdale in England. 



Fam. YII. Opisthocomidae. — The curious and highly special- 

 ized 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 Opisthocojii. 

 Bijffon classed it with the Curassows, P. L. S. Miiller 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 Ealline, and certain points of structure indicate a 

 considerable affinity to the Cuculi. 



The sternimi 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 corrected 

 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 



VOL. IX E 



