Animal Dentition 115 
on the other hand, merely combs off the leaves without injuring the twigs; and the 
more complex crown of its outer pair of lower front teeth is probably to render this 
combing action more complete. The reason why giraffes have lower-crowned cheek-teeth 
than browsing deer is likewise obvious from the foregoing statement, leaves clearly 
requiring less mastication than a mixture of leaves and twigs. One other remark 
before leaying this part of my subject. Ruminants of the modern type are a 
comparatively late development in the earth’s history, their ancestors being animals 
with a full series of teeth of simpler type. Evidently, then, these animals only came 
into being as deciduous trees and grassy plains replaced the evergreens and marshy 
swamps of earlier periods of the earth’s history. Finally, the absence of tusks, as 
effective weapons of offence, is compensated in the ruminants either by the development 
of horns or antlers, or by the speed with which these animals are endowed. 
Space permits of but brief reference to the dentition of the tapirs and rhinoceroses. 
The former are noteworthy as being some of the few hoofed mammals which retain the 
typical number of forty-four teeth, and also from the fact that their whole dentition is 
very like a type that was prevalent during the Tertiary epoch. The molars are low- 
crowned, with simple transverse crests, 
connected in those of the upper jaw 
by a longitudinal outer wall. In the 
rhinoceroses the cheek-teeth are of a 
more complex type, while the front 
teeth are either, as in the Oriental 
forms, reduced in number, or, as in 
the African species, altogether wanting. 
All the Oriental species have one pair 
of lower front teeth developed into 
triangular tusks, projecting almost hori- 
zontally from the jaws and capable of 
inflicting fearful gashes, these animals 
fighting with their tusks, and not with 
their horns. ue 
Although the horses and zebras are ea pes eae roar 
near allies of the tapirs, their dentition is Upper Molar of the sane. 
of a totally different type, being modified 
for a diet of grass, and the molars con- 
sequently having very tall crowns. Indeed, the horse family forms in this respect a 
parallel to the ruminants in the group of cleft-hoofed mammals, the molars (Fig. 14) 
showing four crescents on their grinding-surfaces, although the pattern formed by these 
is quite different from that obtaining in the ruminants. Unlike the ruminants, the horses 
retain the full series of incisors and tusks in both jaws, although the latter are often absent 
in the mare. Moreover, the lower tusk, when present, is quite apart from the incisors, 
standing nearly midway in the gap between the latter and the molars.. From the 
presence of a full series of incisor teeth in both jaws, which close on one another 
like pincers, a horse, unlike a deer or an ox, can inflict a very nasty bite. For the 
same reason these animals can graze much closer than ruminants of the same bodily 
size, and can therefore thrive on pasture where the latter would starve. It is probably 
owing to the absence of either horns or antlers that the members of the horse family 
have retained their front teeth as fighting-weapons, and the difference in this respect 
between these animals and the ruminants (both of which are specially adapted for 
grazing) affords an excellent instance of how the same end is attained by different 
means. 
Figs. 13 and 14. Side view of 
