64 



NORTH AMEEICAN" FAUNA 



[No. 36. 



in profile, as promment accessory tubercles; in other species (chrys- 

 opsis group) the enamel loops sometimes do not reach the outer 

 border of the tooth and the accessory tubercles are often absent or 

 much reduced. 



Remarks. — The subgenus Aporodon, while not sharply set off from 

 Reithrodontomys by any constant e!xternal characters, differs so 

 widely in the molar pattern as described above that its segregation 

 seems desirable. 



It includes several well-marked groups, some of which possess 

 striking characters, both cranial and external, while other species 

 resemble the typical subgenus in all but the tooth characters. 



The tenuirostris-microdon group (including also creper) and the 

 mexicanus group (including miUeri, soderstromi, and gracilis) show the 

 greatest amount of differentiation from typical Reithrodontomys. 

 These agree in having the outer wall of the anteorbital foramen rela- 

 tively narrow (usually much narrower than width of inte'rpterygoid 

 fossa), broad interpterygoid fossa, short palatal foramina, unicolor 

 tail, and dense pelage (rather woolly in most species) of a uniform 

 tawny or ochraceous color, without pronounced darker grizzling. In 

 all species in these groups the subsidiary enamel loops of the upper 

 molars are well-developed and in unworn specimens appear as promi- 

 nent tubercles. 



The chrysopsis group approaches the typical subgenus in having 

 the outer wall of anteorbital foramen broader, interpterygoid fossa 

 narrower and palatal foramina longer. The subsidiary enamel loops 

 of the upper molars are always well developed, but are usually (except 

 in perotensis) not continuous to the outer edge of the tooth and the 

 accessory tubercles are absent or much reduced. The pelage is long 

 and full, and somewhat' more silky than in tenuirostris; the upper- 

 parts are more or less varied with black, and the tail is bicolor. 



R. levipes and R. Mrsutus seem to be aberrant members of the 

 subgenus, agreeing in the character of the pelage with the members 

 of the typical subgenus, but having the subsidiary enamel loops of 

 the upper molars well developed. 



REITHEODONTOMYS LEVIPES GROUP. 



REITHRODONTOMYS LEVIPES Merriam. 



San Sebastian Harvest Mouse. 

 (PI. Ill, fig. 2; PI. VI, fig. 2; PI. VII, figs. 3, 5.) 

 Reithrodontomys levipes Merriam, Proc. Wash. Acad. Sci., Ill, 1901, p. 554. 



Type locality. — San Sebastian, Jalisco, Mexico. 



Distribution. — Known only from the type locality. 



Characters. — About the size of R. fulvescens toltecus; color more 

 decidedly tawny than any members of the fulvescens group ; skull 

 relatively short and broad. 



