1916.] 



SUBGENUS LEUCOCKOSSUEOMYS. 



23 



greatly inflated audital bullae; teeth smaller; nasals broad and trun- 

 cate posteriorly; triangular plate of jugal especially well developed 

 and greatly produced at downward point; posterior border of in- 

 flected angle of mandible nearly at right angle with axis of jaw (in 

 ludovicianus at an angle of about 45° with axis); the indistinct 

 grooves along inner half of face of upper incisors usually more 

 noticeable than in other forms. 



Measurements. — ^Averages of 8 adult males from La Ventura, 

 Coahuila: Total length, 416 (390-430); tail vertebrse, 102 (89-115); 

 hind foot, 63 (59-68.5). Slull: Condylobasal length, 59.3 (58.4- 

 60.5); zygomatic breadth, 44.6 (43-45.6); mastoid breadth, 28.6 

 (27.9-29.5) ; length of nasals, 23 (22.3-23.8) ; length of mandible, 43.8 

 (41.5-45.8); maxillary tooth row, alveoli, 15.9 (15.4-16.3). Females 

 average slightly smaller. For detailed measurements of specimens, 

 see page 35. 



Type specimen. — ^No. Htll U. S. National Museum,- Biological 

 Survey collection. Adult male, skin and skull (teeth moderately 

 worn). Collected March 24, 1891, by C. P. Streator; original number 

 625. Except for the underparts, head, nape, and shoulders, which 

 are in process of renewal, the specimen is in the worn, left-over winter 

 pelage. 



Remarks. — ^This is a well-marked species, easily distinguishable ex- 

 ternally and by skull characters from the other forms of the genus. 

 It is no more closely related to Cynomys ludovicianus arizonensis than 

 to typical ludovicianus, and the southern subspecies arizonensis can 

 not be considered as intermediate in the true sense between it and 

 ludovicianus. In the excessive size of the audital bullaB, mexicanus 

 is the extreme of the arizonensis type, but in other characteristics it 

 more closely resembles ludovicianus, and is separated from both by a 

 number of constant differences. It is completely isolated in its 

 distribution from other forms of Cynomys and occupies only a 

 limited range. 



Specimens examined. — ^Total number, 42, as foUows: 

 Coahuila: La Ventura, 37 (including type); Saltillo, 5.^ 



Subgenus LEUCOCROSSTJROMYS nov. 



Characters. — Size slightly smaUer than in the subgenus Cynomys, 

 general appearance more spermophile-like; tail short, averaging less 

 than one-fifth the total length, and tipped with white; mammse 

 normally 10, rarely 12. SJcuU: General shape of occipital region, 

 viewed from behind, eUiptical-oval; jugal bone weak, thin, and flat, 

 the outer surface at angle of ascending branch only very slightly 

 thickened, the margin rounded, not triangular; maxillary root 



» Collection Mus. Comp. Zool. 



