44 



NORTH AMERICAN- FAUNA. 



[No. 36, 



Measurements. — ^Type: Total length, 183; tail vertebrae, 102; hind 

 foot, 19. One specimen (subadult) from Providencia Mines, Sonora: 

 158.5; 84; 20.5. Average of 4 (adult and subadult) from Casas 

 Grandes, Chihuahua: 165 (157-174); 93 (84-97); 20. SkuU: (See 

 table, p. 81). 



Remarks. — The mice of this group may be distinguished from R. m. 

 megalotis by their greater size, longer tails, and more intensely ochra- 

 ceous coloration. The present form resembles megalotis more closely 

 than do any of the other subspecies and certain specimens of the two 

 forms are colored almost alike on the back. The sides of fulvescens, 

 however, are more extensively buffy and less mixed with brown than 

 those of megalotis. The skulls of fulvescens may usually be distin- 

 guished from those of any member of the megalotis group by the 

 greater breadth of the interpterygoid fossa.^ The ranges of the two 

 species meet along the eastern border of the Sierra Madre in Chi- 

 huahua, but there is no indication of intergradation between them. 



This subspecies was the first member of the group to be described 

 and the third distinctively Mexican species to receive recognition. 

 Although described in 1894, it is still imperfectly known, only a small 

 number of specimens having been collected in the type region. At the 

 time of naming this form Dr. AUen considered it a subspecies of mexi- 

 canus^ and noted its close relationship to the Kio Grande form. Later 

 he accorded it specific rank. The abundant material now available 

 from Mexico shows clearly ihsit fulvescens, tenuis ^ and intermedius are 

 closely related subspecies, connected through central Mexico by a per- 

 fect series of intergrades. The present form apparently intergrades, 

 also, with toltecuSj as indicated by a specimen from Inde, Durango, 

 which agrees with fulvescens in color, but has a long tail like toltecus, 

 and skull intermediate in size between the two forms. The series 

 from Casas Grandes, Chihuahua, agrees with the type series in ex- 

 ternal characters, except for a more pronounced grayish wash on 

 the head and shoulders, but the single adult in the series has a some- 

 what larger skuU than any in the Oposura series. In this character 

 it is matched by a specimen from Parral, Chihuahua. 



Specimens examined. — Total number, 12, from the following locali- 

 ties in Mexico : 



Sonora: Oposura, 3;^ Providencia Mines, 2.^ 

 Chihuahua: Casas Grandes, 5; Parral, 1. 

 Durango: Inde, 1.^ 



1 Skulls oiR. m. saturatus are sometimes difficult to distinguish by this character from those of R.f. tenuis. 



2 The name mcricanus at that time was applied to all the mice of this group from Texas and Mexico. 



3 Collection Am. Mus. Nat. Hist. 

 * Collection Field Mus. Nat. Hist. 

 5 Approaching toltecus. 



