The Mammals of Colorado 
CLASS MAMMALIA 
MAMMALS may be defined as vertebrated or back- 
boned animals in which the blood is constandy pre- 
served at a fixed temperature, as a rule somewhat higher than 
that of the surrounding air; the young are nourished with 
milk provided by the mother from the mammary glands, 
which are situated somewhere on the lower side of the body; 
and the skull is provided with two condyles or swellings 
posteriorly for the attachment of the chain of vertebrae or 
backbone. 
In addition to these characteristics they are (almost in- 
variably) provided with a covering of hair. They have, as 
a rule, four limbs furnished with flat nails, claws, or hoofs. 
Their teeth, when present, are usually separable into three 
categories: incisors, canines, and molars, in which case they 
are described as Heterodont; though in some cases, as in 
the porpoises, when all the teeth are constructed on one 
pattern, they are termed Homodont. Furthermore, the 
teeth of the adult are usually preceded by another set, the 
milk teeth. Finally, the cavity containing the heart and 
lungs is completely separated from that containing the 
viscera by a transverse muscular partition, known as the 
diaphragm or midriff. 
Mammals are divisible into three subclasses, of which, 
