334 THE VERTEBRATE SKELETON. 



The skull of Parrots has some peculiarities. In some 

 Parrots the lachrymal sends back a process which meets the 

 postorbital process of the frontal and completes the orbit. 

 In most birds the upper beak is immovably fixed, but in 

 some it is attached to the cranium, only by the nasals and 

 by flexible processes of the premaxillae, so that by this 

 means a kind of elastic joint is established and the beak is able 

 to be moved on the cranium. In the Parrots and Opisthocomus 

 there is a regular highly movable joint. 



In Cassowaries the fronto-nasal region of the skull is pro- 

 duced into an enormous bony crest, and in Hornbills a 

 somewhat similar structure occurs. Although true teeth do 

 not occur in any known bird except Archaeopteryx, Hesper- 

 ornis, and Iclithyornis, another extinct bird, Odontopteryx, has 

 the margins of both jaws provided with forwardly -directed 

 tooth-like serrations, formed of part of the actual jawbone : a 

 living hawk, ffarpagus, too, has a deeply notched bill, to 

 which correspond serrations in the premaxillae. 



A basipterygoid process of the basisphenoid abuts against 

 the pterygoid in Ratitae and in Tinamous, plovers, fowls, 

 pigeons, ducks and geese among Carinatae, recalling the 

 arrangement met with in many reptiles. The squamosal is 

 sometimes, as in the fowl, united with the postorbital process 

 of the frontal. In the Carinatae the quadrate articulates 

 with the cranium by a double convex surface, in the Ratitae 

 by a single one. The premaxillae are always comparatively 

 large bones, the maxillae on the contrary are small, but 

 give rise to important inwardly-projecting maxillo-palatine 

 processes. 



The relations of the palatines, pterygoids, maxillae, and 

 vomers vary considerably, and on them Huxley has based a 

 classification of birds 1 . In the Ratitae and the Tinamous 

 (Tinamidae), among Carinatae the vomers unite and form a 

 large broad bone, separating the palatines and the pterygoids 

 1 See T. H. Huxley, "On the Classification of Birds," P. Z. S. 1867. 



