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 quadrato-jugal rod ._, ane 
or bar (Fig. 1155, d) ae the maxilary ‘bone ensheathed in horn 
premaxilla (22) upwards and wil aiealbet eu erat Maio: 
forwards. This is a constant fea- TEA Mae ee : 
: : 0, lachrymal bone ; 7, nostril, show- 
ture in recent birds, the degree ing also the articulation of the naso- 
ef motion which this peculiar Premauilary bones ¢. quadrate bone: 
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 vertebre 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 vertebrw in the dor- 
sal region are without the saddle-shaped articulation, and 
are either opisthoccelian, or imperfectly biconcave.” (Marsh.) 
In the fossil Zehthyornis, 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-vertebre of all other birds. The saddle form 
renders the articulation strong and free, and especially 
adapted to motion in a vertical plane. (Marsh.) 
