398 BULLETIN 56, TTNITP:D STATES NATIONAL MUSEUM. 



PEROMYSCUS SONORIENSIS MEDIUS (Mearns). 

 SAN DIEGO PLAINS MOUSE. 



Peromijscus fexanus medius Mearns, Proc. U. S. Nat. Mus., XVIII, 1896, p. 446 (p. 4 of 

 advance sheet issued March 25,1896; original description). — Miller and Rehn, 

 Proc. Bost. Soc. Nat. Kist., XXX, No. l,Dec. 27, 1901, p. 85 (Syst. Results Study 

 N. Am. Mam. to close of 1900). 



Sitomys americanus fhurberi Allen, Bull. Am. Mus. Nat. Hist., V, Art. XII, Aug. 18, 

 1893, pp. 185, 186 (in part). 



[Peromyscns iexensis] medius, Elliot, Field Col. Mus., Zool. Ser., II, 1901, p. 131 

 (Synop. Mam. N. Am.). 



[Peromyscus] thurberi, Elliot. Field Col. Mus., Zool. Ser., IV, 1904, p. 176 (part) 

 (Mam. Mid. Am.). 



Type-locality. — Nachoguero Valley, Lower California, near Monu- 

 ment No. 23S. (Type, skin and skull, Cat. No. 61059, U. S. National 

 Museum.) 



Geograpliical range. — Sonoran and Transition zones of the Pacific 

 Coast Tract. From the Coast Range to the Pacific. 



Description. — In the original description of this 

 form it was said that "between the ranges of the 

 subspecies gamhellii and thurberi, both dark-colored 

 races of Peromyscus sonoriensis, there is a narrow 

 strip of southern and Lower California, bordering 

 the Pacific Ocean for several hundred miles and 

 extending east to the Coast Range of mountains, 

 occupied by the present race, which differs from 

 either of those above mentioned in being paler, 

 with more drab and clay-colored tints. It is smaller 

 Fig. 77.-PEROMYSCUS ^-hau P. s. gamhellii, with much larger ears and a 

 SONORIENSIS MEDIUS. shortcr tall. Its paler, more ochraceous coloration 

 ^^^^^- at once distinguishes it from P. s. thurberi, with 



which it agrees in size. From P. s. deserticola of the interior deserts, 

 it differs in being less robust and in having somewhat larger ears and 

 much darker color." The upper surface is deep wood-brown, shading 

 to russet on the cheeks, sides, and rump, and thickly mixed with black 

 above, giving a distinctly dusky vertebral area; ears clove- brown, 

 densely coated with hair, with a faint hoary edging; top of head 

 wood -brown, paler than the back; under surface of body and feet 

 pure white; tail, black above, white on sides and beneath. Mammae, 

 three pairs (P. {, A. -^ I. |). The skull is shown in fig. 77. 



Remarks. — The San Diego plains mouse frequents the seashore 

 and the dampest parts of the mountains and plains of the Pacific 

 Coast Tract. At Jacumba Hot Springs it was abundant, and four 

 pregnant females were taken from May 18 to 23, 1894, the young 

 numbering 3, 5, 6, and 6, respectively. Young were taken in traps 

 late in May. At San Isidro, Lower California, a female contained 

 three fetuses as late as June 29. 



