1. Infraorbital foramen very large, larger than foramen magnum: 2
1'. Infraorbital foramen smaller than foramen magnum: 3
2. Dorsal hairs in part modified to form stout spines: Erethizontidae, Erethizon dorsatum.
2'. Dorsal hairs not so modified: Myocastoridae (Echimyidae), Myocastor coypus.
3. Tail dorsoventrally flattened, broad and naked: Castor canadensis.
3'. Tail not dorsoventrally flattened: 4
4. Infraorbital foramen a vertically elongate V-shaped slit; cheek teeth 3/3: 5
4'. Infraorbital foramen not as above; cheek teeth more than 3/3: 6
5. Cusps of molars in three longitudinal rows; tail mostly naked: Muridae
5'. Cusps of molars in two longitudinal rows; tail haired: Cricetidae
6. Infraorbital foramen a very small canal or round hole; tail, if elongate, not naked and scaly: 7
6'. Infraorbital foramen an ovoid hole; hind feet elongate; tail elongate, naked, and scaly: Family Dipodidae, Zapus.
7. External fur-lined cheek pouches present; prominent postorbital processes lacking: 8
7'. External fur-lined cheek pouches absent; prominent postorbital processes present: Family Sciuridae.
8. Skull heavy, zygomatic breadth greater than mastoid breadth; tail less than three-quarters head and body length; claws on forefeet at least somewhat enlarged for digging: Family Geomyidae.
8'. Skull light; zygomatic breadth less than mastoid breadth; tail more than three-quarters head and body length; claws on forefeet not greatly modified for digging: Family Heteromyidae.
Key modified from Findley (1987).
Last Update: 2 Nov 2007