66 LAND MAMMALS IN THE WESTERN HEMISPHERE 



different mammals, the massiveness of the arch being pro- 

 portional to the power of the jaws. One of the principal 

 muscles of mastication (the masseter) is attached to the zygo- 

 matic arch. 



The squamosal itself is a large plate, which makes up a great 

 part of the side-wall of the cranium and articulates above with 

 the frontal and parietal ; it also supports the lower jaw, the 

 articular surface for which is called the glenoid cavity. The 

 lower jaw is held in place by the postglenoid process, which is 

 a projection, usually a transverse ridge, behind the cavity. 

 Back of the postglenoid process is the entrance to the middle 

 ear, the auditory meatus, which may be merely an irregular hole, 

 or a more or less elongated tube. The meatus is an opening 

 into the tympanic, a bone which at birth is a mere ring and in 

 some mammals remains permanently in that condition, but 

 as a rule develops into a swollen, olive-shaped auditory bulla, 

 which sometimes reaches enormous proportions, especially 

 in nocturnal mammals. The labyrinth of the internal ear is 

 contained in the periotic, a very dense bone which is con- 

 cealed in the interior of the cranium, but in many mammals 

 a portion of it, the mastoid, is exposed on the surface between 

 the squamosal and occipital. 



The lower jaw-bone {inferior maxillary, or mandible) is the 

 only freely movable element of the skull ; it consists of two 

 halves which meet anteriorly at the chin in a contact of greater 

 or less length, called the symphysis. In nearly all young 

 mammals and in many adult forms the two halves of the lower 

 jaw are separate and are held together at the symphysis only 

 by ligaments, while in others, as in Man, they are indistinguish- 

 ably fused to form a single bone. Each half consists of two 

 portions, a horizontal part or ramus and an ascending ramus 

 or vertical part ; the former supports all of the lower teeth, and 

 its length, depth and thickness are very largely conditioned by 

 the number and size of those teeth. The ascending ramus is 

 a broad, rather thin plate, divided at the upper end into two 



