THE HAR OF MAMMALS. 563 
In the elephant the upper incisors form the tusks, the cor- 
responding teeth of the lower jaw being absent. In many 
teeth, as those of the deer (Fig. 490), the 
crown of the molars is quite convex, with 
crescent-shaped enamel areas. The canines 
are large and sabre-shaped in the cat fam- 
ily, while in the pigs, especially the baby- 
roussa of Malaysia, the upper pair curve 
upward and backward to the forehead. 
The premolars and molars have two or pig, 490,--crown of 
three roots or fangs ; in none of the lower [h¢ {ot of a acer, 
vertebrates do the teeth have more than grescents.— After 
one root. 
The organs of sense are much developed, especially the 
ear. The quadrate bone of ‘the reptiles and birds, which is 
\ ee R 
Fig. 491.—Diagram of the labyrinth of the ear in J, the fish, 17, the bird, and ZZ, a 
mammal. JU, utriculus; S, sacculus; US, utriculus and sacculus; Cr, canalis reuniens ; 
R, recessus labyrinthi; UC, commencement of the cochlea,-0, Z, lagena; AK, cceca 
sac at the apex; C, coecal sac of the vestibulum of the cochlear canal.—After Wai- 
deyer, from Gegenbaur. 
large, external, and suspends the lower jaw to the skull, 
now becomes much changed, and forms the zygomatic 
process of the squamosal bone. The labyrinth of the ear, 
largest in fishes, is smallest in mammals. The cochlea’ 
