STRUCTURE OF BIRDS. 521 



calcareous shell ; there is an amnion and allantois, and no 

 metamorphosis after hatching. 



The external form of birds is very persistent ; the different 

 parts of the body have been named in terms of continual use 

 in descriptive ornithology. Hence, without entering into 

 details, we reproduce from Coues's "Key" his figure of the 

 topography of a bird. 



The student, after a careful study of the external form, 

 should prepare a skeleton of the common fowl, or examine one 

 already at hand, and observe those characters peculiar to birds. 

 The skull is formed of bones consolidated into a more roomy 

 brain-box than in any reptiles, unless it be the Pterosaurians. 

 In the parrots the beak of the upper jaw is articulated (Fig. 

 453, n) to the skull, so that the movement of the beak on the 

 skull is unusually free. The 

 quadrate bone (Fig. 453, e) is 

 usually movable on the skull ; 

 and in the parrots when the 

 mouth opens the upper jaw rises, 

 since when the mandible is low- 

 ered, the maxillo - iugal rod 



/T^- ^ro 7\ i Fi S- 458- Skull of Parrot: 22, pre- 



Ol* bar (X lg. 45o, /) pushes the maxillary bone ensheathed in horn ; 



/n^\ i -i 15 ' nasal bones ; v, mandible, the 



(22) Upwards and end sheathed with horn- /, malo- 



forwards. This is a constant fea- 

 ture in recent birds, the degree 

 of motion which this peculiar 

 mechanism allows being variable. Owen - 



The form of a bird's vertebrae is peculiar to the class ; the 

 articulation of the body (centrum) in all the vertebrae in 

 front of the sacrum being saddle-shaped. "In Strigops 

 and a few other land birds ; in the penguins, the terns, and 

 some other aquatic birds, one or more vertebrae in the dor- 

 sal region are without the saddle-shaped articulation, and 

 are either opisthocoelian, or imperfectly biconcave." (Marsh.) 

 In the fossil Ichthyornis, which had a powerful flight, the 

 vertebrae are bi-concave, as in fishes, and Amphibians, and 

 a few reptiles ; but the third cervical shows an approach to 

 the saddle - vertebrae of all other birds. The saddle form 

 renders the articulation strong and free, and especially 

 adapted to motion in a vertical plane. (Marsh.) 



