BIRDS, 225 



Works on Reptiles for further reference. 



44 Smithsonian Reports " ; " Tropical Nature," Wallace ; " List of 

 Reptiles and Batrachians near Springfield, Massachusetts," Allen ; 

 Holbrook's " Herpetology of North America " ; Agassiz, " Embryology 

 of Turtles," in " Contributions to Natural History of the United 

 States " ; " Check List of North American Reptiles and Batrachians,'" 

 E. D. Cope ; " Serpents," " Popular Science Monthly," vol. iv. 



Class VI. BIRDS (Avcs). 



General Characteristics. From the reptiles we pass to 

 the birds, that may be characterized as warm-blooded 

 feathered Vertebrates, having the fore-limbs adapted for 

 flight, the jaws inclosed in horny beaks, and the bones 

 hdllow. 



Skeleton. In examining the skeleton of a bird (Fig, 

 268), we first notice its extreme lightness. This is due to 

 the fact that many of the bones that are filled with mar- 

 row in other animals are in the birds hollow air-chambers. 

 The skull-bones in the adult bird form a single piece, and 

 except in certain extinct forms (Fig. 274) the jaws are 

 toothless and inclosed in horny cases called beaks. The 

 lower jaw is not joined directly to the skull as in man, but to 

 a quadrate bone (Fig. 268, q), as in the reptiles and batra- 

 chians. The neck, consisting of from nine to twenty-four 

 vertebrae, is extremely long and flexible, so that the bird 

 can trim its feathers on any part of the body ; a knot can 

 almost be tied in the neck of the flamingo. The (dorsal) 

 vertebrae, from six to ten in number, are firmly joined 

 (anchylosed) in the flying birds ; but in the non-fliers, as 

 the emu and ostrich, they are movable one upon another. 

 The vertebrae between the dorsal and those constituting 

 the tail (caudal) are joined, forming a single bone called 

 the sacrum, which, joined with the innominate bones, forms 

 the pelvic arch to which is attached the first bone of the 

 leg or thigh (Fig. 268, th}. To the thigh or femur is at- 

 tached the tibia, to which a small fibula is joined. The 



