2 The Mammals of Colorado 
however, only the first is represented in the fauna of 
Colorado. These are: 
1. The Eiitheria, containing by far the greater number 
of mammals; in these the young are nourished before birth 
by means of a placenta; they are found all over the world. 
2. The Metatheria, containing the Marsupials, the young 
of which are born in a very rudimentary condition in con- 
sequence of the absence of a placenta, an organ by which the 
young are nourished while still unborn. These animals are 
found in Australia and America; two species of the well 
known Opossum are found in the eastern and southern 
portions of the United States, but the animal has not yet 
been met with in Colorado. 
3. The Prototheria, containing the egg-laying Echidna 
and Ornithorhyncus, confined to the Australian region. 
Out of the nine orders into which the Eiitheria are gener- 
ally divided only five are represented in Colorado; the 
Primates (Monkeys), Edentata (Ant-eaters), and naturally 
the two marine orders Cetacea and Siren ia being absent. 
The following key to the five Colorado orders is founded 
on the most obvious and easily recognized but by no means 
the most fundamental characters, and does not necessarily 
hold good for other than Colorado animals. 
A. Fore limbs not modified for flight. 
a. Feet provided with claws or sometimes flat nails. 
a'. Incisor teeth not chisel-shaped, or if so, more than two 
in number above and below. 
a". Upper lip projecting beyond the lower; middle pair 
of incisors larger than the others. Insectivora. 
b''. Upper lip not projecting; middle pair of incisors not 
larger than the others. Carnivora. 
b'. Incisors chisel-shaped; in the upper usually, and in the 
lower jaw invariably two in number only. Rodentia. 
b. Feet provided with solid hoofs in which the last joint of the 
toe bone is embedded. Ungulata. 
B. Fore limbs modified for flight; fingers elongated to support a 
membranous wing. Chiroptera. 
