218 NEW ZEALAND NATURE-STUDY BOOK 
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danger they expose the white underside of the tail which 
thus acts as a guide and signal to others. | When startled 
or puzzled, they often thump the ground smartly with 
their strong hind feet in order to warn others who may 
be in the neighbourhood. They breed with great rapidity, 
it being estimated that from a single pair as many as a 
million descendants would spring in the course of five 
years, but for the many dangers to which they are exposed. 
THE SKELETON. 
The skull consists of a number of bones firmly fused 
together. At the sides are two large sockets for the eyes, 
and further back two flask-shaped cavities for the internal 
ears. The upper jaw is firmly fixed to the brain-case, but 
the lower jaw is hinged to the bony arches below the eyes. 
Each jaw bears teeth in a row of sockets. On each side of 
the upper jaw are two incisor teeth, and further back six 
molar or grinding teeth. The lower jaw carries on each 
side a single incisor and five grinding teeth, and can be 
moved both up and down and from side to side—as in the 
case of all plant-eaters. 
The vertebral column, or back-bone, is made up of a 
chain of small bony rings or vertebree, which extends from 
the back of the skull to the extremity of the tail; each 
vertebra consists of a solid central portion, from which 
spring several short processes differing in length in different 
parts of the vertebral column. On.the front surface of the 
first vertebra, called the atlas, are two hollows into which 
two knobs on the skull are fitted. In the neck there are 
seven vertebree—a number which remains constant in most 
mammals. Next to these come the chest vertebre which 
carry the ribs, and further. back still the large stout 
abdominal vertebr. Between these and the tail-bones 
—indeed separating the two sets of bones—are four 
