— 1901-1902.] The Teeth of Fishes. 275 
in the upper, where they remain buried in the dense gum. In 
_ the narwhal two teeth alone persist in the upper. In the female 
these are rudimentary; in the male, one tusk, in some rare 
_ instances both tusks, continue to grow from a persistent pulp. 
_ The tusk is quite straight, and is marked by spiral grooves, 
winding from left to right. Where both teeth have been 
developed, the spiral grooves wind in the same direction. 
Whalebone whales have no teeth, but in their place they 
_ have baleen plates placed transversely to the axis of the 
mouth, but not at right angles to it. The plates are fringed, 
and as the whale takes large mouthfuls of water containing 
minute marine creatures, these are caught by the baleen 
plates, while the water is expelled. 
_ Cheiroptera, or bats, The bats possess wings, and are 
divided into two groups—namely, insectivorous and frugivor- 
ous. Insectivorous bats have the following dental formula— 
2; 643 pm. 2; m. % 
Blood-sucking vampires (Desmodus) have teeth specially 
_ adapted for their blood-sucking habits. 
Rodentia. The animals of this order are pretty well scattered 
over the world. They have long chisel-shaped incisors, which 
‘grow from persistent pulps. In hares and rabbits there is 
_ an extra pair of incisors, which grow behind the large ones. 
_ Proboscidea. In this group the incisors grow from per- 
sistent pulps. The molars are massive teeth, and are made 
up of plates held together with cementum. The tusks represent 
the principal ivory of commerce, and, on account of their 
elasticity, are largely used in the manufacture of billiard- 
balls. This elasticity is due to the small size of the 
dentinal tubes, and to the frequent bends which these 
make, giving to ivory the peculiar engine - turning pattern 
that distinguishes it from bone. In the elephant there is 
‘no vertical succession of teeth, as one finds in most 
- mammals, but they come from behind and travel forwards 
as the front ones drop out. 
_ Ungulata, or hoofed mammals, These are divided into 
Perissodactyle or odd-toed, and Artiodactyle or even-toed. 
The Perissodactyle comprise the horse, tapir, palzotherium, 
and rhinoceros. The dentition of the horse is— 
33445; pm, 4; m 2 
