1. Occlusal surface of each molar with a complicated pattern of triangular and other shaped figures outlined in enamel enclosing dentine lakes; occlusal surface flat: 2
Left: Lower Microtus tooth (Arvicolinae), m1, anterior to left. Right: Upper Neotoma teeth (Neotominae), M1-3, anterior to left
1'. Occlusal surface of each molar with a series of bunodont cusps, which may disappear with wear, leaving a few relatively formless dentine lakes bordered with enamel; occlusal
surface usually not flat: Subfamily Neotominae (part), 16
Upper Peromyscus teeth, M1-3, anterior to left
2. Dorsal border of infraorbital foramen forming a posteriorly directed notch when viewed from above; incisive foramina longer than occlusal surface
of molars and about as wide as distance between molar rows: Subfamily Sigmodontinae and Neotominae (part), 3
2'. Dorsal border of infraorbital foramen not as above; incisive foramina shorter than occlusal surface of molars, narrower than distance between molar rows: Subfamily
Arvicolinae, 9
3. Molars with V-shaped reentrant angles; without conspicuous temporal ridges on parietal bones; ears prominent and naked; color pattern sharply bicolored: Neotoma, 4
3'. Molars without V-shaped reentrant angles; with conspicuous temporal ridges on parietal bones; ears partly hidden in fur and haired; color not sharply bicolored: Sigmodon, 30
4. Tail conspicuously bushy: Neotoma cinerea.
4'. Tail not conspicuously bushy: 5
5. Dorsal color steel gray in adults; nasal septum (plate extending dorsally between palatine foramina) intact: Neotoma micropus.
5'. Dorsal color brown or tan in adults; nasal septum with a conspicuous posterior emargination: 6
6. Hairs of throat plumbeous basally (rarely a patch of hairs white to the base); anterointernal reentrant angle of upper M1 extending more than halfway across anterior lobe of tooth: Neotoma mexicana.
6'. Hairs of throat white basally; anterointernal reentrant angle of upper M1 extending less than halfway across anterior lobe of tooth: 7
7. Tail with moderately long hairs, sometimes only vaguely bicolored; lingual fold of M3 as deep as or deeper than labial fold; nasals usually truncate posteriorly; anterolateral dentine tract well developed: Neotoma stephensi.
7'. Tail with very short hairs, always sharply bicolored; lingual fold of M3 shorter than labial fold; nasals sharply pointed posteriorly; anterolateral dentine tract not extending more than 2 mm above base of lateral reentrant groove: 8.
8. Occurring west of the Rio Grande: Neotoma albigula
8'. Occurring east of the Rio Grande: Neotoma leucodon
9. Tail laterally compressed, long, scaly; condylobasal length more than 60 mm: Ondatra zibethicus.
9'. Tail rounded, hairy; condylobasal length less than 35 mm: 10
10. Cheek teeth rooted: 11
10'. Cheek teeth not rooted: 12
11. Dorsal color reddish: Myodes gapperi.
11', Dorsal color grayish or brownish: Phenacomys intermedius.
12. Tail about one-third or more of total length: Microtus longicaudus.
12'. Tail less than one-third total length: 13
13. M3 with four dentine lakes: Microtus ochrogaster.
13'. M3 with five dentine lakes: 14
14. M2 with four dentine lakes and a posterior accessory loop: Microtus pennsylvanicus.
14'. M2 with four dentine lakes but no posterior accessory loop: 15
15. Incisive foramina abruptly constricted posteriorly; color dark; tails of adults usually more than 35 mm: Microtus montanus.
15'. Incisive foramina broad and truncate posteriorly; color brown; tails of adults seldom attaining 35 mm: Microtus mogollonensis.
16. Upper incisors with a groove on anterior face: Reithrodontomys, 17
16'. Upper incisors without a groove: 19
17. Occlusal pattern of last lower molar S-shaped; occipitonasal length usually more than 22 mm; dorsal color usually rich golden brown: Reithrodontomys fulvescens.
17'. Occlusal pattern of last lower molar C-shaped; occipitonasal length usually less than 21 mm; color not golden brown: 18
18. Braincase breadth less than 9.7 mm; tail relatively short (usually less than one-half total length) and with narrow, distinct dorsal stripe: Reithrodontomys montanus.
18'. Braincase breadth more than 9.6 mm; tail longer (usually more than one-half total length), often not sharply bicolored, and dorsal stripe broader: Reithrodontomys megalotis.
19. Occipitonasal length of skull less than 22 mm; length of head and body less than 65 mm; length of hindfoot usually less than 20 mm: Baiomys taylori.
19'. Occipitonasal length of skull more than 22 mm; length of head and body more than 65 mm; length of hindfoot usually 20 mm or more: 20
20. Coronoid process relatively high and recurved; tail length less than 60 percent of length of head and body: Onychomys, 21
20'. Coronoid process relatively low, not strongly recurved; tail length more than 60 percent of length of head and body: Peromyscus, 23
21. Tail less than half the length of head and body; crown length of maxillary tooth-row 4.0 mm or more: Onychomys leucogaster.
21'. Tail more than half the length of head and body; crown length of maxillary tooth-row 3.9 mm or less: 22.
22. Posterior edge of palate concave or truncate; distribution west of continental divide near the New Mexico/Arizona border and to the west: Onychomys torridus
22'. Posterior edge of palate slightly convex to well developed spine; geographic distribution from Animas Valley and northwestern Chihuahua east through central and southern New Mexico: Onychomys arenicola
23. Two principal labial reentrant angles of first and second upper molars with accessory tubercles or enamel loops: 25
23'. Two principal labial reentrant angles of first and second upper molars without (or, at best, with rudimentary) accessory tubercles or enamel loops: 24
24. Tail well-haired and tufted; fur long and soft; nasals exceed premaxillae slightly or not at all: Peromyscus crinitus.
24'. Tail with extremely short hairs, almost naked, not tufted; nasals decidedly longer than premaxillae: Peromyscus eremicus.
25. Tail tufted, with terminal hairs that are conspicuously longer than others: 27
25'. Tail terete, that is, with no conspicuously elongate terminal hairs: 26
26. Hindfoot length 21 mm or less; greatest length of skull less than 27 mm; tail sharply bicolored: Peromyscus maniculatus.
26'. Hindfoot length more than 21 mm; greatest length of skull more than 27 mm; tail often not sharply bicolored: Peromyscus leucopus.
27. Ear usually 20 mm or less: 28
27'. Ear usually 20 mm or more: 29
28. Ankles white: Peromyscus pectoralis.
28'. Extension of dark dorsal coloration extending distally over ankle: Peromyscus boylii.
29. Length of ear longer than length of hindfoot on fresh specimens; tail length usually shorter than head and body length: Peromyscus
truei.
[Some populations formerly recognized as P. truei in southwestern New Mexico have been recognized as a distinct species, P. gratus.]
29'. Length of ear equal to or shorter than length of hindfoot; tail usually longer than head and body length: Peromyscus nasutus.
30. Venter ochraceous: Sigmodon fulviventer.
30'. Venter not ochraceous: 31
31. Snout, eye-rings, and sometimes upper forelegs yellowish or orangish, total length usually less than 260 mm: S. ochrognathus.
31'. Snout, eye-rings, and upper forelegs not as above; total length often more than 260 mm: S. hispidus.
Key modified from Findley (1987).
Last Update: 2 Nov 2007