MAMMAI>S OF THE MEXICAN BOUNDARY. 339 



are SO intimately eomieeted with tlu» preceding suhtainiiy (''^(■n//-i/i^) 

 that the division into two groups is purely a matter of eoin enience. 

 The}^ are confined to the Pahearctic and Nearctic regions. {J^Z/nver and 

 Lydekker. ) 



Genus CYNOMYS Rafinesque (1817). 



Five distinct claws to each foot, that on the thumb as large as that 

 on the tifth toe. Cheek-pouches shallow. Ear-conchs small. Tail 

 short. Skull stout, with postorbital process strongly developed and 

 directed outward. Dentition as shown in fig. 56. First premolar 

 nearly as large as the second. Lateral rows of teeth strongly con- 

 vergent posteriorly and twisted on their axis. Incisor teeth white or 

 but slightly colored. Mammaj, four or five pairs. Form stout and 

 muscular. 



Two species of this genus inhal)it the region of the Mexican Border. 

 Their ranges are complementary, Cynoitvys ludoviclanus arisoneii-sis 

 being an inhabitant of the Sonoran Life Zone, on both sides of the 

 International Line, while 0. gunmsoni is restricted to the Transition 

 Zone, its range to the southward falling short of the Mexican Boundary. 



CYNOMYS LUDOVICIANUS ARIZONENSIS (Mearns). 

 ARIZONA PRAIRIE-DOG. 



t'iinoinijK arizonensis Meahnw, Bull. Am/ Mu8. Nat Hist., II, No. 4, Feb. 21, 18H0, 

 p. 305 (original description). — Allen, Bull. Am. Mus. Nat. Hist., VII, 1895, 

 p. 237.— Miller and Refix, Proc. Bost. Soc. Nat. Hist., XXX, No. 1, Dec. 27, 

 1901, p. 57 (Syst. Results Study N. Am. Mam. to close of 1900). 



[Cifnonti/s] nrizone}isi'<, Elliot, Field Col. Mus., Zool. !^er., 11, 1901 (Syiiop. Mam. 

 N. Am.); IV, 1904. p. 154 (Mam. Mid. Am.). 



Ciinomys ludovicianus arizonensis, Merriam, Proc. Biol. f?oc. Wash., VII, p. 158, 

 July 27, 1892. 



Type-locality. — Point of Mountain, near Willcox, Cochise Coiuity, 

 Arizona. (Type, No. VsW, American Museum of Natural History. 

 New York.) 



Geographical range. — Grassy plains of the Sonoran Zone, from the 

 Pecos River, Texas, west to the valley of the San Pedro River, Arizona. 



DcHcription. — Large.st of the prairie-dogs found in the United States. ' 

 Length, -1:00 mm.; tail vertebra?, 90; hind foot, 64; head, 73. Skull, 

 66 by 4:5; incisors white or slightly colored. Mammte -i pairs (A |, 

 P. [, A. [, I, \). In winter the pelage of the upper surface is pale sandy 

 butf, with a few all-black and iiiany gray-tipped hairs; under surface 

 white or pale j^ellowish; sides yellow; tail with a narrow, subterminal 

 bar of black; untler side of tail and inguinal region tawny ochraceous; 

 underfur copious. In summer the imderfur is wanting; color vinace- 

 ous cinnamon above, with but few black hairs and grizzling obsolete; 

 below whitish; sides jellowish. Iris hazel. 



"The Cynomys mexicanus of Merriam, from I^a Ventura, Coahuila, Mexico, is al)()ut 

 the same size, but has a much longer tail. 



