E. melanogaster species group. Described by Thomas (1921g) as a species but subsequently included either in E. melanogaster (Hinton, 1923; Ellerman, 1941, 1961; Ellerman and Morrison-Scott, 1951; Corbet, 1978c; Musser and Carleton, 1993; Pavlinov et al., 1995a) or E. miletus (Corbet and Hill, 1992). Comparisons of Anthonyís (1941) large AMNH sample, obtained at the type locality of cachinus and vicinity, with E. miletus and E. melanogaster proper confirm Thomasí view that cachinus is a distinctive species. Adult E. cachinus are large-bodied and long-tailed, with soft, long and thick fur, the upperparts bright tawny brown and the underparts gray washed with hues ranging from pale buff to ochraceous (the only species in the E. melanogaster group with such bright venters). They are about the same body size as the large E. miletus, but have longer tails (mean = 50.9 mm, range = 43-61 mm, n = 105) than E. miletus (43.6 mm, 35-48 mm, n = 39) and E. melanogaster (36.1 mm, 31-42 mm, n = 34); no other species in the E. melanogaster group has such a long tail. Compared with E. miletus, cranium of E. cachinus is slightly smaller in length and width dimensions and strikingly much lower in profile (flattened compared with the high, arched and stocky cranium of E. miletus); M3 has four lingual salient angles (like E. miletus), but also four labial angles in 95 of 109 specimens (three in E. miletus). Occlusal pattern consists of a single anterior lamina, two inverted chevron-shaped laminae, and an irregularly-shaped heel, while all other forms have only two anterior laminae and a heel with an angular labial projection. Thomas (1921g) clearly described the M3 pattern in cachinus, and Hinton (1923) used it to separate cachinus from other forms of melanogaster in his key to Eothenomys.
We synonymize the NW Yunnan confinii with E. cachinus in contrast to its peripatetic association under E. eleusis (G. M. Allen, 1940; Zhang et al., 1997), E. melanogaster (Corbet and Hill, 1992), E. mucronatus (Kaneko, 2002), or E. miletus (Ye et al., 2002). Hintonís (1923) description of the pelage and tail length (59 mm) matches that of E. cachinus, his cranial differences between confinii and miletus precisely distinguish E. cachinus from E. miletus, and confinii too was obtained west of the Salween River Valley adjacent to the region where Anthony (1941) worked. Kaneko (2002) relegated both cachinus and confinii, along with miletus, to E. mucronatus (here considered a junior synonym of E. melanogaster; see below). Examination of the same specimens studied by Kaneko (2002) supplies no morphological evidence for considering the three taxa as members of a single species. Specimens of E. cachinus (including confinii) and E. miletus can be consistently separated by cranial conformation and relative tail length (comparing specimens of similar age), as previously indicated.
In NE Burma, Anthony (1941) encountered E. cachinus (identified as E. melanogaster cachinus) in heavy forest, 2300-3200 m, and found the smaller E. melanogaster (reported as E. m. libonotus) at lower elevations, 1200-2750, in open habitats (meadows, near cultivation, and shrubby growth along streams). This region of Burma also harbors two endemic murines, Niviventer brahma (Musser, 1970b, 1973a) and an undescribed species of Niviventer (Musser and Lunde, ms). Curiously, Zhang et al. (1997) listed cachinus both as a separate species and as a synonym of E. melanogaster. The specimens that Ellerman (1961) recorded from N Burma are E. melanogaster.