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 

 lower part of the skull are two rounded "ear bones" known 

 as the audital bullce. 



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. 



XIX 



