ZOOLOGICAL POSITION AND STRUCTURE 2 i 
In the adult condition there are normally, as in the 
ox, six pairs of these teeth in each jaw, of which the 
three foremost are simpler in structure than the others, 
and are preceded by milk-teeth, these being called 
premolars. The last three pairs, on the other hand, 
have no deciduous predecessors, and are known as 
molars. Taking first the premolars, the crowns of 
these teeth may be described as consisting of two 
vertical columns, one on the outer and the other on 
the inner side, the latter being crescentic in form, 
with the horns of the crescent directed outwardly. 
The upper molars are more complicated, and consist 
of two pairs of somewhat smaller but practically 
similar columns, with the crescents of the inner pair 
likewise directed outwardly. In the upper molars 
the surface of the crown is either nearly square or 
oblong, and the height may, as in the ox, be very 
considerable ; while the varying hardness of the three 
constituents of which the tooth is formed makes a 
rough and irregular surface admirably adapted for 
masticating grass, twigs and leaves, or grain. The 
central portion of each column is formed internally 
by a comparatively soft substance termed dentine, 
externally to which is a layer of hard flinty enamel, 
while in the hollows between and around the 
columns is spread the third constituent, known as 
cement. When a tooth of this type is gradually 
worn away by grinding against the teeth of the lower 
jaw, it v/ill be obvious that the surface will present 
alternations of these harder and softer materials. In 
all the Pecora the cheek-teeth develop roots, and are 
then incapable of further growth ; and it may be 
d-ssumed, prima facie, that the higher the crowns of 
these teeth, the longer the life of their owner, although 
