46 



NORTH AMERICAN FAUNA. 



[No. 36. 



Color. — ^Upperparts and sides rich ochraceous-buff ; head and 

 shoulders strongly ochraceous, without trace of gra3dsh; back more 

 heavily mixed with black than in fulvescens; ears slightly darker; 

 im4erparts usually grayish white, sometimes faintly tinged with buff; 

 tail hair-brown above, grayish white below. 



SlcuTl. — Closely similar to that oi fulvescens. 



Measurements. — Average of 3 adults from Mazatlan, Sinaloa: Total 

 length, 169 (163-177); tail vertebrae, 98 (96-102); hind foot, 20.8 

 (20.5-21). Average of 5 adults from Chacala, Durango: 177; 105; 

 21.3. Skull: (See table, p. 81). 



Remarlcs. — This is the most widely distributed member of the 

 fulvescens group and over its extensive range shows remarkably little 

 variation. Specimens from opposite sides of the continent can not 

 be distinguished by characters either of skin or skull. There is con- ' 

 siderable individual variation in both color and cranial characters, but 

 none which is correlated with distribution. 



The subspecies intergrades wit}! fulvescens in southern Sonora, with 

 intermedius in Nuevo Leon and Tamaulipas, with difficilis in Vera 

 Cruz and with toltecus in Michoacan. Direct comparison with the 

 type has not been possible, but specimens in the Biological Survey 

 Collection from Mazatlan, forwarded to Mr. G. S. Miller, jr., while he 

 was in London, Were compared by him with the type in the British 

 Museum and found to agree very closely with it. ]\dr. Miller states: 

 ^'I should say there is no question of the identity of the Mazatlan 

 specimens with tenuis.''^ In making comparisons these specimens 

 have been chiefly used. 



Specimens from Ameca, Jalisco, forming the basis of "griseofiavus/^ 

 show in comparison with tenuis a very slight cranial difference, con- 

 sisting of a more inflated braincase. In this character they resemble 

 Tielvolus, although on geographical grounds they should be approach- 

 ing toltecus. In color and size they agree closely with tenuis. The 

 type is decidedly more grayish on the back and less intensely buffy 

 on the sides than is usual in this subspecies, but two topotypes agree 

 perfectly with the Mazatlan specimens of tenuis. A series from Cha- 

 cala, Durango, is a little darker and more richly colored, as well as 

 slightly larger than the typical form. Specimens from the arid coast 

 region of Vera Cruz (Carrizal, Santa Maria, and Catemaco) are clearly 

 intermediate between tenuis and dificilis, the ears and tails being 

 noticeably darker than in tenuis but the underparts and backs paler 

 than in difficilis. 



Specimens from as far north as Cerro de la Silla and Santa Catarina, 

 Nuevo Leon, are referable to tenuis, though those from Santa Cata- 

 rina might almost as weU be considered intermedius. Specimens 

 from Acambaro, Michoacan, are intermediate in size between tenuis 

 and toltecus. 



