Introduction 
have only the opossum. Of the higher orders, the Sirenia are 
represented by the few remaining manatees of Florida, the FEden- 
tata only by a species of armadillo which crosses into Texas from 
farther south. Proboscidea (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 
RVili 

