SruUGTUBE OF BIRDS. 



531 



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

 metamoriiliosis after hatcliing. 



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. 



Tlie student, after a careful study of tlio external form, 

 should jirepare 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 tlie jiarrots the beak of the upper jaw is articulated (Fig. 

 453, 11) 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 ojiens the upiper jaw rises, 

 since when the mandible is Ioav- 

 ered, the maxillo - jugal rod 

 or bar (Fig. 453, J) pushes the 

 premaxilla (22) upwards and 

 forwards. This is a constant fea- 

 ture in recent birds, the degree 

 of motion which this peculiar 

 mechanism allows being variable. 



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

 articulation of the body (centrum) in all the vertebris 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 vertebrse in the dor- 

 sal region are without the saddle-shaped articulation, and 

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

 In the fossil Ichfliyornis, 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 - vertebras of all other Ijirds. The saddle form 

 renders the articulation strong and free, and especially 

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



Fig 4!3 —Skull of Piriiit 22, pre- 

 maxillary bone ensheatlK-d in horn ; 

 15, nasai bones ; v, mandible, the 

 end sheathed with horn- /, malo- 

 squamosal zygomatic style or max- 

 illo-jugal bar ; g, post-frontal bone ; 

 0, lachrymal bone ; n, nostril, show- 

 ing also the articulation of the naso- 

 preniaxillary bone; e, quadrate bone ; 

 m, orbit ; 1, occipital bone.— After 

 Owen. 



