CRANIAL CHARACTERS.--Skull short in relation to width, deep relative to other dimensions, somewhat convex; delicate, papery, without strong angularity; braincase relatively unexpanded; antorbital foramen obliquely oval and transmits masseter muscle of great size; foramen in inferior ramus of zygomatic process of maxillary for passage of superior maxillary branch of trigeminal nerve small; zygomata not wide-spreading; underside of zygoma nearly horizontal, upper edge anteriorly rises prominently owing to extension of jugal upward along maxillary; jugal and lachrymal in contact; one ramus of zygomatic process of maxilla arises directly above other; rostrum thick basally and relatively attenuate distally; ends of nasals project noticeably beyond incisors; premaxillaries develop strong alveolar plate separating superior incisors for half their length; palatal bones shortened posteriorly, free edge often concave; incisive foramina long, broad, and separated by bulbose (except at posterior end) bony septum; mastoid bullae absent; auditory bullae short and transversely placed; postorbital process never present; parietals nearly square, sometimes emarginate in front; angle of mandible flattened and bent inward; coronoid process weak, acute, and slopes strongly upward. DENTAL CHARACTERS.--Dental formula I 1, C O, P 1, M 3 -- -- -- -- -- -- -- -- = 18; i 1 c o p o m 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