BIRDS. 225 
Works on Reptiles for further reference. 
“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.—BirpDs (Aves). 
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 
hollow. 
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, ¢), as in the reptiles and batra- 
chians. The neck, consisting of from nine to twenty-four 
vertebre, 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) 
vertebree, 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 vertebre 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, #2). To the thigh or femur is at- 
tached the tibia, to which a small fibula is joined. The 
