ON THE ANATOMY OF THE HOATZIN. 469 
related to the Tinamide, Gallinz, or Rallidxe, from which it will be 
remembered the Cuculide differ in that they lack the oil-gland tuft, 
and the Musophagide in that they have no colic ceca. Opisthocomus 
being holorhinal,* can have nothing to do with the Charadriiform 
birds. In the Rallide there is only a single posterior notch on 
each side of the carina sterni, at the same time that a crop is never 
developed. These features, when correlated with the peculiarities of 
the palate, remove them from the necessity of further consideration. 
Opisthocomus must therefore, from what has been just shown, be 
a Gallinaceous bird, or form a group by itself. As there is no 
Gallinaceous bird without a direct articulation between the pterygoid 
bones and the basisphenoidal rostrum, it is hardly possible to include 
the Hoatzin along with them; and yet it resembles them most closely, 
as it does the Cuculide, in the length of its colic ceca and the number 
of its rectrices. It is not far removed from the Musophagide as well. 
All these facts can be expressed as follows :— 
Page 114. 
Tufted oil-gland; 
colic czeca ; 
no basipterygoid 
facets. 
Ancestral type. 
MusoPHaciID2. 
Colic czca 
lost. 
Diagram to show the position of Opisthocomus. 
This diagram indicates that the Galliform ancestor, besides giving 
rise to the at this moment irrelevant Rallide and Psittaci, varied 
also in a strictly Gallinaceous direction, the ancestor of Opisthocomus 
* “ Proceedings of the Zoological Society,” 1873, p. 33. (Supra, p. 124.) 
