Key to the Orders of U.S. Mammals

1 a. Epipubic bones present (a pair of bones extending forward from the pubes); fur-lined belly pouch in females; incisors 5/4
b. Epipubic bones not present; no belly pouch; incisors fewer than 5/4
Marsupialia
2
2 a. Teeth homodont; often vestigial or absent
b. Teeth heterodont
3
4
3 a. Forelimbs modified into flippers; hind limbs absent externally
b. Forelimbs not modified into flippers; hind limbs present
Cetacea
Edentata
4 a. Forelimbs modified for flight; fingers longer than forearm, supporting membranous wing
b. Forelimbs not modified for flight; fingers shorter than forearm and without flight membrane
Chiroptera
5
5 a. Hind limbs absent
b. Hind limbs present
Sirenia
6
6 a. Feet unguligrade; hooves present
b. Feet not unguligrade; digits with claws or nails
7
8
7 a. Axis of foot passing between almost equally developed 3rd and 4th digits
b. Axis of foot passing through 3rd digit
Artiodactyla
Perissodactyla
8 a. Canines absent; anterior and posterior teeth separated by a wide gap
b. Canines present; anterior and posterior teeth not separated by a wide gap
9
10
9 a. Incisors 2/1, the 2nd small and set immediately behind the 1st
b. Incisors 1/1
Lagomorpha
Rodentia
10 a. Orbit enclosed by a bony ring formed by junction of postorbital process of frontal with zygomatic arch
b. Orbital not enclosed by complete bony ring
Primates
11
11 a. Canines small, no larger than incisors
b. Canines large, much larger than incisors
Insectivora
Carnivora

Orders and common name examples:

Vertebrate Key