Vv OPISTHOCOMIDAE 241 
this formation, but from the Upper Miocene of Oeningen and the 
Pliocene of Attica, in the latter of which Gallus accompanies it. 
Meleagris has been discovered in the Miocene of Colorado, and 
the Post-pliocene of New Jersey; Gallus in the Pliocene of 
France, Palaeotetriv and Pedicecetes in that of Oregon; and, 
finally, bones of Lagopus have been brought to light in the - 
French Plistocene, and those of 7’etrao urogallus at Kent’s Hole 
near Torquay and in the caves of Teesdale in England. 
Fam. VII. 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 OpIsTHOcOMI. 
Buffon classed it with the Curassows, P. L. S. Miller 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, beg 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 shghtly 
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 R 
