350 SYSTEMATIC ZOOLOGY. 



ing thigh-bones nine feet in length and vertebrae five feet 

 across. The Pterodactyls were flying reptiles with w T ings 

 like those of the bats, except that the wing-membrane was 

 supported by a single finger. 



CLASS II. AVES (BIRDS). 



No one can have the slightest question as to whether a 

 certain animal is a bird or not. The feathers, the fore- 

 limbs fitted for flight, and the horny, toothless beak are 

 characteristic of all living forms. 



Feathers arise from the outer layer or epidermis of the 

 skin, and each has its tip inserted in a pit or follicle in the 

 integument. Feathers vary considerably. Most promi- 

 nent are the large, strongly built contour feathers, which give 

 the animal its general shape. Beneath these are the down 

 and the pin-feathers. Feathers are not uniformly dis- 

 tributed over the body, but are gathered in feather tracts, 

 the arrangement of which varies in different birds. The 

 feathers are not permanent structures, but they are molted 

 or shed and replaced by a new growth, this taking place 

 usually once a year. In connection with the feathers 

 should be mentioned the oil-glands (the only glands in the 

 skin of birds) upon the tail, the secretion of which is used 

 in preening the feathers. 



In their origin feathers are much like the scales found 

 on the feet, and are probably modifications of such struc- 

 tures. The scales on the feet may be small or broad, both 

 kinds sometimes occurring on the same foot. The spur of 

 the cock is but an extremely developed scale with a bony 

 core. These scales, like those of reptiles, differ from those 

 of fishes in that they are developed on the outside of the 

 outer layer of the skin (compare p. 318). The toes are 



