HOACTZIN, or HOATZIN, a bird of tropical South America, thought by Buffon to be that indicated by Hernandez or Fernandez under these names, the _Opisthocomus hoazin_ or _O. cristatus_ of modern ornithologists--a very curious and remarkable form, which has long exercised the ingenuity of classifiers. Placed by Buffon among his "_Hoccos_" (Curassows), and then by P. L. S. Muller and J. F. Gmelin in the Linnaean genus _Phasianus_, some of its many peculiarities were recognized by J. K. W. Illiger in 1811 as sufficient to establish it as a distinct genus, _Opisthocomus_; but various positions were assigned to it by subsequent systematic authors. L'Herminier was the first to give any account of its anatomy (_Comptes rendus_, 1837, v. 433), and from his time our knowledge of it has been successively increased by Johannes Muller (_Ber. Akad. Wissensch. Berlin_, 1841, p. 177), Deville (_Rev. et mag. de zoologie_, 1852, p. 217), Gervais (Castelnau, _Exped. Amerique du Sud, zoologie, anatomie_, p. 66), Huxley (_Proc. Zool. Society_, 1868, p. 304), Perrin (_Trans. Zool. Society_, ix. p. 353), and A. H. Garrod (_Proc. Zool. Society_, 1879, p. 109). After a minute description of the skeleton of _Opisthocomus_, with the especial object of determining its affinities, Huxley declared that it "resembles the ordinary gallinaceous birds and pigeons more than it does any others, and that when it diverges from them it is either sui generis or approaches the _Musophagidae_." He accordingly regarded it as the type and sole member of a group, named by him _Heteromorphae_, which sprang from the great Carinate stem later than the _Tinamomorphae_, _Turnicomorphae_, or _Charadriomorphae_, but before the _Peristeromorphae_, _Pteroclomorphae_ or _Alectoromorphae_. This conclusion is substantially the same as that at which A. H. Garrod subsequently arrived after closely examining and dissecting specimens preserved in spirit; but the latter has gone further and endeavoured to trace more particularly the descent of this peculiar form and some others, remarking that the ancestor of _Opisthocomus_ must have left the parent stem very shortly before the true _Gallinae_ first appeared, and at about the same time as the independent pedigree of the _Cuculidae_ and _Musophagidae_ commenced--these two groups being, he believed, very closely related, and _Opisthocomus_ serving to fill the gap between them.