(52 COMPARATIVE ZOOLOGY. 
Fig. 26.— Mouth of the Crocodile: d , tongue; e , glands; /, inferior, and g, superior, 
valve, separating the cavity of the mouth from the throat, h. 
have a slender forked tongue, consisting of a pair of mus¬ 
cular cylinders, which is solely an instrument of touch. 
Birds are without lips or teeth, the jaws being covered 
with horn forming a beak. Tin’s varies greatly in shape, 
being extremely wide in the Whippoorwill, remarkably 
long in the Pelican, stout in the Eagle, and slender in the 
Hummer. It is hardest in those that tear or bruise their 
food, and softest in water-birds. The tongue is also cov¬ 
ered with a horny sheath, and generally spinous, its chief 
function being to secure the food when in the mouth. 
It is proportionally largest and most fleshy in the Parrots. 
The main characteristics of the mammalian mouth are 
flesh lips and mobile cheeks. 23 In the duck-billed Mon- 
otremes lips are wanting, and in the Porpoises they are 
barely represented. But in the herbivorous quadrupeds 
they, with the tongue, are the chief organs of prehension ; 
in the carnivorous tribes they are thin and retractile; 
while in the Whale the upper lip falls down like a cur¬ 
tain, overlapping the lower jaw several feet. As a rule, 
the mouth is terminal; but in the Elephant, Tapir, Hog, 
