Introduction 



have only the opossum. Of the higher orders, the Sirenia are 

 represented by the few remaining manatees of Florida, the Eden- 

 tata only by a species of armadillo which crosses into Texas from 

 farther south. ProboscLica (elephants) are entirely lacking, and 

 of Primates our only native representatives are the Indian and 

 Eskimo. Of the remaining orders we have an abundance of species. 



In the scientific study of mammals we are compelled to 

 make use of more or less obscure characters, and when separa- 

 ting species, we are unable to base descriptions entirely upon 

 the external appearance, as is possible in the case of birds. 



Some mammals, especially among the mice, exhibit scarcely 

 any external differences, while an examination of their skulls 

 and teeth shows that they belong to quite different genera. 



Indeed, few mammals are very brightly marked, doubtless 

 due to their general nocturnal habits and their need of protec- 

 tive colouration. 



The necessity of studying some of the skeletal characters in 

 identifying mammals makes it desirable to have an idea of the 

 more important portions of their bony structure. While there is 

 no reason why the structure of any particular portion of an 

 animal's anatomy should be regarded as of more importance than 

 another in studying its relationship, it is nevertheless a fact that 

 in every group of animals certain organs or parts of the skeleton 

 show a greater susceptibility to modification, and thus furnish a 

 much easier clue to the origin and development of the species, 

 than is offered by those parts in which there is very slight 

 modification. Thus in the mammals it is the structure of the 

 skull, the teeth and the lower leg and foot bones that furnish 

 the basis for most of our classification. 



The Skull. The skull is really composed of a large number 

 of bones, each of which has a distinctive name, but in the 

 adult animal they have become so firmly joined together that 

 even the lines of juncture are nearly obliterated, and we may 

 therefore say that the adult skull consists of two parts the 

 skull proper and the lower jaw or mandible, the latter being 

 separable into two symmetrical halves. The skull proper consists 

 of the bony box or brain case, the back of which is known as 

 the occipital bone, and in it is the round hole or foramen through 

 which the spinal chord joins the brain. The forward part of 



XVlll 



