520 PROCEEDINGS OF THE NATIONAL MUSEUM vol. no 



phyllotine or sigmodont affinities. A more precise determination 

 of the systematic position of Hesperomys simplex cannot be made 

 on the basis of available data. 



Characters 



External characters: ^odj heavy, limbs short and adapted for 

 arboreal life; size from approximately that of a house mouse (Mus 

 musculus) to that of a medium-sized house rat (Rattus rattus) ; pelage 

 long and thick; dorsal surface from buffy to tawny or rufous, with a 

 fine to coarse mixture of dark brown; a poorly defined dark middorsal 

 band sometimes present; underparts from sharply defined white to 

 dark gi'ay, bufT, or ochraceous little or not at all defined from 

 sides; an ochraceous lateral line present or absent; ea,rs comparatively 

 small, length (from notch) approximately one-half to four-fifths length 

 of hind foot (dry, with claw); tail never sharply bicolor, sparsely 

 haired except at thickly furred base, tip with or without a conspicuous 

 tuft; length of tail from 40 to 60 percent of total length; hind foot 

 comparatively short and broad, sole at heel bare but half hidden by 

 overlapping hairs of side of tarsus; six plantar tubercles present and 

 well developed; first hind toe with claw extending to base of middle 

 phalanx of second toe, fifth hind toe well developed for grasping, its 

 claw nearly or quite on a level with base of terminal phalanx of fourth 

 toe; claws short and recurved, the fifth exposed, the others partially 

 concealed by the digital tufts. 



Cranial characters (pis. 1-3): Skull moderately heavy; sides of 

 supraorbital region ridged or beaded and markedly divergent pos- 

 teriorward ; midtransverse width of paired frontals more than greatest 

 width of rostrum; nasals comparatively short; upper anterior corner 

 of vertical zygomatic plate rounded and slightly or not at all visible 

 when skull is viewed from above; zygomatic plate comparatively 

 slender, its anteroposterior width at midpoint not more than alveolar 

 length of M^'^; interparietal v>^ell developed, its median length vary- 

 ing from approximately three-tenths to slightly more than one-half 

 its transverse width; sphenopalatine vacuities small, slit-hke, or ob- 

 solete; incisive foramina moderately long and well opened, length 

 more than one-half but less than nine-tenths that of diastema, the 

 posterior borders terminating from slightly before to slightly behind 

 anterior plane of first molars; auditory bullae small, their antero- 

 posterior length, exclusive of tube, less than that of molar row. 



Dental characters (fig. 3; pi. 4a,b): Upper incisors recurved; 

 upper molar rows parallel-sided; inner and outer halves of unworn to 

 moderately worn upper molar crowns distinctly crested, of lower 

 molars bilevel, i.e., with outer cusps lower than inner cusps; major 

 fold well open, a low enterostyle present at its base in M^'^, and fre- 



