Type genus–Delanymys Hayman, 1962. Definition–very small-bodied (head and body less than 65 mm in adults, 5-7 grams), arboreal muroid with a thinly haired (caudal hairs less than two tail scales long), very long (twice as long as head and body), and semiprehensile tail (conformation of body and appendages similar to Sulawesian Haeromys; see Musser, 1990); four digits and rudimentary fifth on front foot (fifth very short, middle two long, second longer than fifth), five on hind foot (first short, middle three very long, and fifth two-thirds as long as middle digits and opposable), sharp and moderately curved claws on all digits; hind foot very long (relative to body size) and narrow, with four interdigital pads, thenar and hypothenar elongate and subequal in length; pelage short, dense and soft (not silky) with short guard hairs; four pairs of teats; cranium small and delicate with very short and narrow rostrum compared with the large, globular, and smooth (faint temporal ridges) braincase; interorbit narrow and without ridges, postorbit broad; zygomatic plate narrow, with slightly projecting anterior margin (very shallow zygomatic notch), indistinct masseteric tubercle; alisphenoid strut absent; large subsquamosal foramen bounded ventrally by prominent hamular process; incisive foramina long but not extending past anterior margins of molars, posterior palatine foramina at level of posterior half of M1; bony palate wide, with posterior inverted V-shaped margin at level of second molar; mesoterygoid fossa wide and open to level of second molar, sphenopalatine vacuties large; parapterygoid fossae long, wide, moderately excavated; bullae small; carotid circulation fully derived (minute stapedial foramen, no squamosal-alisphenoid groove or sphenofrontal foramen; pattern 3 as per Carleton, 1980, and Voss, 1988); soft palate with three premolar and four intermolar ridges; mandible short and robust, pronounced coronoid process, incisor alveolus projects prominently between coronoid and condyloid processes; upper incisors ungrooved, slightly compressed, orthodont or slightly opisthodont; lower incisors narrow; molars brachyodont with three-rooted uppers and two-rooted lowers; primary cusps large, arranged in two rows, connected by short longitudinal mure and transverse enamel crests; anterocone(id) complete on M1 and m1(appears bifurcate on m1 with wear because anteroconid coalesces with anterolophid); accessory lingual cusp large, next to protocone on M1 and M2 and confluent after wear; anteroloph wide and short (style-like), mesoloph a long and prominent ridge reaching cingular margin on both M1 and M2; anterolophid large (comma-shaped) on m1 and m2, mesolophid smaller but connects with style on cingulum; posterior cingular ridge (posteroloph) connects posterior margins of metacone and hypocone on M1 and M2, large posterior cingulum on m1 and m2; M2 subequal to M1, M3 more than half as long as M2 with similar occlusal pattern (based upon study of specimens in AMNH and information in Delany [1975], Dieterlen [1969b], Hayman [1962a], and W. Verheyen [1965b]; see dental terminology in Carleton and Musser [1989]). Contents–Delanymys Hayman, 1962; Stenodontomys Pocock, 1987.

The discoverer Hayman (1962a) regarded Delanymys as a dendromurine, morphologically close to Petromyscus. According to Lavocat (1964:184), however, "Delanymys shows typical Cricetid dental structures, particularly in the presence of the longitudinal crest. Thus, Petromyscus and Delanymys are perfect structural links between Mystromys and the typical Dendromurinae and show us the clear systematic affinities of these forms." W. Verheyen (1965:12) was similarly convinced that "Delanymys represents in its tooth structure an almost perfect morphological link between Mystromys and the Dendromurinae." F. Petter (1967b) treated Delanymys and Petromyscus as cricetids, but united the two in Petromyscinae, divorcing them from dendromurines because he thought the shared lingual cusp to be nonhomologous; he identified it as the protocone and the mesial cusp as neomorphic, having developed from the longitudinal crest (mure). Jaeger (1977b) discounted Petter’s hypothesis and reemphasized a cingular origin for the accessory cusp, noting that lingual accessory cusps are found in some Miocene myocricetodontines as well as dendromurines; because occlusal patterns are otherwise so different among myocricetodontines, dendromurines, and petromyscines, he viewed their shared lingual cusps as parallelisms reflecting an origin from a distant common ancestor.

Carleton and Musser (1984:332) enumerated the general characters of Petromyscus and Delanymys, as Petromyscinae, but pointed out that, aside from the lingual accessory cusps, "no other derived features are shared by the two that would indicate they belong in a natural group separate from other African cricetids. Even the configuration of the lingual cusps and the extent of their attachment to the protocones are different in the two genera." This view draws support from Denys (1994c), who described the dental similarities between Delanymys and Stenodontomys (type species originally named as a Mystromys by Lavocat, 1956) from the Plio-Pleistocene of South Africa (Pocock, 1987) and Namibia (Senut et al., 1992); she further contrasted their molar patterns with Petromyscus and regarded the Petromyscinae as an unnatural group, the genera originating independently from an early Miocene lineage different from that leading to dendromurines. Accessory lingual cusps are common to cricetomyines, dendromurines, leimacomyines, some Miocene myocricetodontines, deomyines and murines (cusp t4), as well as delanymyines and petromyscines, and apparently were independently acquired in several Old World muroid groups.