14 



Obs. This may just possibly be H. mexicanus of De Saussure, 

 but I cannot so identify it, and prefer to err on the safe side, if at 

 all. Besides the above specimen, I have others from Tehuacan 

 and Tehuantepec. It is nothing like any of the foregoing North 

 American species. A female shows three pair of mammae — one 

 axillary, two inguinal. 



Subgenus Ontchomys, Baird. 



Syn. Hypudmus, sp., Maxim., Reise, ii. 1841, 99, necauct. Mus, sp., Aud. 

 & Bach., Q. N. A. ii. 1851, 327 (missourietisis), Hesperomys subg. 

 OnycTiomys, Baird, M. N. A. 1857, 458 (type Hypudmus leucogaster, 

 Maxim.). 



Diag. Skull strictl}^ as in Hesj)eromys ( Vesperimiis), but mo- 

 lars larger with sharper saliencies and re-entrances. Coronoid 

 attaining level of condyle. External form somewhat approaching 

 the arvicoline in stoutness, and in shortness of tail and ears. 

 Tail in tj'pe of the genus not one-half the length of the body 

 alone — little if any longer than the head, very thick and tapering 

 to an obtuse point. Ears about as in the arvicoline genus Evoto- 

 mys {Hypudseus of Baird). Fore feet verj- large, f to ^ as long 

 as the hinder, with long, little curved, almost fossorial claws, 

 longer than those of the hind feet. Soles quadrituberculate only 

 — densely furry to the tubercles. White beneath, as in typical 

 Vesperimus. A strongly differentiated form ! To the type of this 

 section {Hypudeeus leucogaster of Maximilian, afterward 3Ius 

 missou7'ie7isis of Audubon) I add a peculiar variety — more proba- 

 bly a second species. The two may be thus distinguished: — 



n. Tail much less than half the head and body, scarcely twice the hind foot. 

 Fore foot more than half the hind foot. Ear about 0.50 high. Be- 

 neath snow-white ; above mouse-brown with darker dorsal area. 



LEUCOGASTER. 



b. Tail nearly half the head and body, about 2^ times the hind foot. Fore 

 foot only half the hind foot. Ear about 0.75 high; beneath tawny 

 white ; above brownish fulvous without darker dorsal area. 



(var ?) TORRIDUS. 



7. Hesperomys (Onychomys) leucogaster, (Maxim.) Baird. 

 Syn. Hypudmus leucogaster^ Maxim., Eeise, ii. 1841, 99 (Fort Clark). 

 Hesperomys {Onychomys) leucogaster, Bd., M. N. A. 1857, 480. 

 (Nebraska.) 



Mus missouriensis, Aud. & Bach., Q. N. A. ii. 1851, 337, pi. 100. 

 (Fort Union, Montana.) 



