266 THE BIRDS 



which are divided one from another merely by a thin bony partition. 

 The nostrils may be rounded or slit-like, and may be divided by 

 a septum, when they are termed impervious, or there may be no 

 division between them, when they are termed pervious nostrils. 

 The beak is covered with a horny sheath, which is composed of 

 one or more pieces ; it varies in texture, and sometimes has curious 

 outgrowths, as in the sheathbills and hornbills. The beak varies 

 considerably in length and form. It may be fine and slender, 

 short and thick, hook-shaped, wedge-shaped, spoon-shaped, straight 

 and flat, curved upwards or downwards, adorned with knobs, 

 horns, or shields, and the edges may be notched, serrated, lobed or 

 festooned. The many different and peculiar shapes of the birds' 

 bills have been gradually brought about by the requirements of 

 the particular species, and, broadly speaking, are intimately related 

 with the nature of their food, though this does not altogether 

 account for the grotesquely shaped bills which many birds possess. 



The bones of the skull become completely united early in life, 

 showing no trace of their original division, and the skull is joined 

 to the neck by a single hinge, a condyle, as is the case in the reptiles. 



The backbone of a bird differs from that of a mammal. The 

 neck division is remarkably flexible, and the vertebrae of which 

 it is composed articulate one to another by saddle-shaped surfaces, 

 and vary in number from eight to twenty- three. In the middle, 

 or thoracic, portion the vertebrae in many instances are fused 

 together into a solid bar of bone. The tail, in all living birds, 

 has very few vertebrae. They number from eight to ten, the last 

 one being very large, and from its curious shape called the " plough- 

 share-bone," or Py go style ; to this are attached the tail feathers. 

 It is an interesting fact that in very young birds this bone is 

 made up of several distinct vertebras, which later on become fused 

 together, showing that originally birds must have possessed longer 

 caudal appendages, marking their descent from the lizard-tailed 

 birds of bygone ages. 



The foot of a bird is encased in a scaly covering, and is very 

 rarely feathered. The number of toes (digits) in the foot may 

 be three or four ; a fifth toe is never present, while in the ostrich 

 the number is reduced to two. The foot shows considerable 

 modification in different species. Birds frequenting forests and 

 leading a strictly arboreal existence, such as parrots, cuckoos, 



