Introduction 
the skull comprises the upper jaw, the nasal bones, surrounding 
the nostrils, and the large eye sockets. The bones forming the 
roof of the mouth constitute the palate and those forming the 

Skull and one side of mandible of Musk Rat. 
N nasal. F frontal. P parietal. O occipital. Z zygomatic arch. B audital bulla 
Mx maxillary. PMx premaxillary. I incisors. M molars. 
CP coronoid process. CD condyle. A angle. 
forehead are the frontals, while on the posterior portion of the 
iower part of the skull are two rounded ‘‘ear bones” known 
as the audital bulle. 
The Teeth.—The teeth of mammals are divided into four 
groups, the incisors or cutting teeth placed across the front of 
the jaws, the canines, four rather elongated teeth placed at the 
front corners of the jaws, two above and two below, the pre- 
molars placed immediately behind the canines, and back of these 
the molars or grinders. Most mammals have two sets of teeth; 
the milk teeth and the permanent teeth. The former are weaker 
and are only retained during the early years of the animal’s life 
when they are succeeded by the permanent set. The premolars 
are represented in the milk dentition, but the molars are not, and 
that is the reason for separating them. In structure, however, they 
are quite similar and it is often impossible to distinguish them. 
The simplest form of tooth is a_ single-pointed cone, 
such as we see in the toothed whales; all canine teeth are 
similar to this in structure, while the incisors are generally 
more flattened and sometimes slightly lobed. 
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