Key to the species of Arborimus
The following key was published by Adam and Hayes in 1998:
1 Reddish orange to cinnamon on dorsum; tail thick, well haired and not strongly bicolored; incisive foramina wide; upper incisors strongly recurved; a pronounced longitudinal depression between orbits . . . . 2
Dark brown on dorsum; tail slender, scantily haired and distinctly bicolored; incisive foramina narrow; upper incisors not strongly recurved; longitudinal depression between orbits not pronounced. . . . Arborimus albipes
2 Larger overall, nasals extend further posteriorly relative to the maxillaries, lacks an abductor digiti secundi muscle and lacks a full complement of lumbricales muscles; typically exhibits a diploid number (2n) = 52, with 22 pairs of acrocentric or subtelocentric autosomes, 3 pairs of small submetacentric autosomes, a large submetacentric X, and a small metacentric Y chromosomes; some individuals exhibit a diploid number (2n) = 48 in which four acrocentric pairs replace the two large metacentric pairs (Johnson and George, 1991); occurring in western forests of Oregon . . . . Arborimus longicaudus
Smaller overall, nasals do not extend further posteriorly relative to the maxillaries, presence of an abductor digiti secundi muscle and a full complement of lumbricales muscles; exhibits a diploid number (2n) = 40 or 42 with 5 pairs of large metacentric autosomes, 1 pair of large submetacentric autosomes, 11 pairs of small acrocentric or subtelocentric autosomes, 2 pairs of small submetacentric autosomes, and a small subtelocentric Y chromosome; individuals with 42 chromosomes differ from those with 40 in having one less pair of large metacentric autosomes and two additional pairs of acrocentric or subtelocentric autosomes (Johnson and George, 1991); occurring in coastal forests of northwest California . . . . Arborimus pomo
References
Adam MD, Hayes JP. 1998. Arborimus pomo. Mammalian Species (593):pp.1-5.
Johnson ML, George SB. 1991. Species limits within the Arborimus longicaudus species-complex (Mammalia: Rodentia) with a description of a new species from California. Contributions in Science (Los Angeles), 429:1-16.