INTRODUCTION 7 



top of the snout is covered by the two nasal bones {Na), which are set into the 

 front of the frontals, bounded on the sides by the maxillae and usually 

 premaxillae, and cover the olfactory chamber. In the olfactory chamber of all 

 mammals, except whales and Sirenia, occur very thin, closely i oiled, and 

 fenestrated films of bone, covered with a mucous membrane and better developed 

 in forms with an acute sense of smell. These are the turbinals, and are 

 regarded as highly developed processes of the ethmoid bone. They are usually 

 developed in several groups, those on the right and left being separated by the 

 lamina perpendicularis of the ethmoid. The turbinals which occupy the 

 upper portion of the olfactory cavity are termed the ethmoturbinals {ET), 

 while those in the lower portion of the cavity, attached to the maxilla, are 

 termed the maxilloturbinals (MT). 



The back of the lower surface of the skull is composed of the basioccipital 

 and basisphenoid bones. From the basisphenoid and alisphenoid there projects 

 downward a short vertical plate of bone, the pterygoid (pt), which makes the 

 sides of the posterior nasal passage, where the floor is formed by the soft 

 palate. In front, the pterygoid unites with the back of the palatine (pi), the 

 front of which bone expands to make a horizontal plate which, uniting with 

 the corresponding plate of the maxilla, completes the hard palate, which is the 

 floor of the olfactory chamber. 



The lower jaw consists of two symmetrical and diverging halves, which 

 meet in front along a rough surface (symphysis), and are either held together 

 by connective tissue, or are rigidly fused in old age by bone. The upper or 

 alveolar border is usually occupied by a row of teeth, the lower border is 

 convex and thickened, rising in the region of the symphysis in front, and 

 making an abrupt angle («) behind, which may be prolonged into a process, 

 or bent inward. Behind the last tooth the upper margin rises, as a rule, 

 into a high compressed coronoid process, to which the temporal muscle 

 is attached, while the powerful masseter muscle is provided with an 

 attachment in the shape of a shallow depression on the outer side of the 

 coronoid. 



The posterior margin of the ascending portion of the lower jaw is expanded 

 just above the angle to make a thickened convex articular condyle (cd), which 

 fits the glenoid fossa on the squamosal, and may be elongated transversely or 

 longitudinally. A wide alveolar canal extends the length of the lower jaw, 

 and carries the mandibular artery and nerve from which arise numerous 

 branches to the various teeth. On the inner side of either jaw may occur 

 a shallow groove {sulcus mylohyoideus), which is especially characteristic of 

 marsupials. 



The brain cavity in mammals is markedly larger than in the other classes 

 of vertebrates. In general the different orders show a measurable increase in 

 the size and structure of the brain, so much so that Owen used this as a basis 

 of his systematic arrangement. The fact first observed by Marsh, that size 

 of the brain is universally smaller among Eocene genera than among the 

 members of the same families in later Tertiary and later times, is very 

 noteworthy ; and indeed the brain cavity in the skulls of some of the giant 

 Eocene amblypods can be contained within the medullary canal. 



Development and Morphology of the Skull. — As in reptiles and birds, the skull 

 consists of a primordial portion preformed in cartilage, and a secondary 

 portion, the bones of which develop in membranes. The various bones are at 



