Dental characters.—Dental formula
I 1 | C O | P 1 | M 3 | = 18; | |||||||
| , | , | , | |||||||||
i 1 | c o | p o | mv 3 |
upper incisors short, compressed, curved backward, and strongly grooved; lower incisors slender, curved backward, and ungrooved; both upper and lower incisors deep orange or yellow; four upper cheek-teeth present; premolar small, single rooted and, sometimes, non-functional; upper molars tri-rooted, sub-hypsodont, and with occlusal surface non-cuspidate (flat); enamel pattern, much complicated, consisting of one main re-entrant fold lingually and four re-entrant folds labially; three lower molars, bi-rooted, sub-hypsodont, flat crowned, with two outer and four inner re-entrant folds.
Postcranial characters.—Neck short and weak; atlas large; axis separate from atlas; remaining (5) cervical vertebrae also free; thoracic (12) and lumbar (7) vertebrae strongly built; posterior lumbars with enlarged neural and anteriorly directed transverse processes; sacral vertebrae (7) as in murids; caudal vertebrae variable in number (average 36); clavicle long, slender, uniformly curved, convex outwardly; scapula with supraspinous and infraspinous fossae of equal size; forelimbs short, approximately half as long as hind limbs; hind limbs elongate, slender; femur with third trochanter; tibia and fibula fused slightly distal to middle of former; five elongate, separate metatarsals (first and fifth subequal, shorter than others).
ARTIFICIAL KEY TO THE SPECIES OF THE GENUS ZAPUS
| A. | Baculum with tip spade-shaped and tip wider than 0.43 mm; underfur with medullary pattern rectangular, cuticular scales small; coronoid process of mandible long and slender, angle of divergence from condyle broad; angle of mandible turned in and wide; pterygoid fossae wide; skull broad in relation to length; premolars with crescentine fold on occlusal surface. | |
| Zapus trinotatus [p. 385] | ||
| A´. | Baculum with tip lanceolate (not spade-shaped) and tip less than 0.43 mm wide; underfur with medullary pattern square or rectangular; but, if rectangular, cuticular scales large; coronoid process short and broad, angle of divergence from condyle narrow; angle of mandible turned inward and small to medium; pterygoid fossae usually narrow; skull not broad in relation to length; premolars without crescentine fold on occlusal surface. | |
| B (A´). | Baculum less than 5.1 mm in total length; guard hair averaging 115 micra in diameter; underfur with rectangular medullary pattern, cuticular scales large; skull small; incisive foramina shorter than 4.6 mm; condylobasal length averaging less than 20 mm; length of maxillary tooth-row averaging less than 3.7 mm; palatal breadth at M3 less than 4.2 mm. | |
| Zapus hudsonius [p. 420] | ||
| B´. | Baculum more than 5.1 mm in total length; guard hair averaging more than 140 micra in diameter; underfur with square medullary pattern, cuticular scales moderately large; skull large; incisive foramina longer than 4.7 mm; condylobasal length more than 21 mm; maxillary tooth-row averaging more than 3.8 mm; palatal breadth at M3 more than 4.4 mm. | |
| Zapus princeps [p. 394] | ||