VERTEBRATA. 217 



The characters of birds are so unvarying " that it is difficult to 

 separate them into subordinate groups;" these are "more 

 arbitrary and artificial than in those of the other vertebrate 

 classes." 



Bill without teeth (all living birds). 

 Breast-bone keeled. 

 Feet not webbed. 



Legs feathered to the knees. 

 Nostrils pierced in the bony structure of the bill. 

 Bill various, not arched from the base. 



Tongue long, extensible PICI. 



Tongue short, not extensible. 

 One toe behind. 



Feet short, weak VOLITORES. 



Feet long, moderately strong... PASSERES. 



Two toes behind SCANSORES. 



Bill strongly arched from the base. 



Two toes behind PSITTACI. 



One toe behind ACCIPITRES. 



Nostrils pierced in a membrane. 



Hind toe on a level with the others ... COLUMB^E. 



Hind toe elevated GALLING. 



Legs naked above the knees GRALI^E. 



Feet webbed ANSERES. 



Breast-bone not keeled STRUTHIONES. 



Bill with teeth. Extinct. (Odontornithes.) 



Breast-bone keeled ODONTORM^E. 



Breast-bone not keeled ODONTOLC^E. 



Breast-bone rudimentary SAURURE. 



There are about 8000 species of birds according to Schmarda 

 (1878), but 11,162 are enumerated in G. B. Gray's ' Hand-List ' 

 (1871) ; of these, about 270 species, not counting stragglers, are 

 British. 



Order I. PTCI. 



SAGITTILINGUES. CELEOMORPH^E. SAUROGNATILE. 

 Bill straight, wedge-shaped : tongue extensible, barbed at the 

 end. Tail-feathers stiff at the points. Feet short, stout ; first 

 and fourth toes turned backwards. 



The bill in the typical species is of an ivory-like hardness and 

 much compressed, especially towards the tip. The tongue, pro- 

 vided with a tenacious secretion, is capable of great elongation 

 and extension, the two cornua of the hyoid bone extending round 



