MAMMALS OF THE MEXICAN BOUND AR-X. 



387 



rate index to our knowledge of the steps in the evolution of recent 

 species. From this standpoint I have lately re-examined the members 

 of what has been designated the " sonoriensis group," and have failed 

 to trace any connection between them and P. leucopus, through inter- 

 grading forms. 



On the contrary, comparison of series of specimens of P leuccpus 

 from Minnesota with the form nebrascensis occupying the adjacent 

 plains of Dakota, shows the two to be very distinct in both external 

 and cranial characters. Similar comparisons of specimens from Kan- 

 sas with those from Nebraska, and of series from Indian Territory 

 with series from Oklahoma, furnish no evidence of intergradation. 

 The series of P. leucopus from Minnesota to Indian Territory shows 

 but little geographical variation; while the campestrian forms on 

 the west show a wide range of variation. Indian Territory furnishes 

 nearly typical examples of P. leucopus; ' but these specimens — ^from 

 Fort Cobb — are much more closely related to the P. texanus than 

 to P. s. blandus from the same region. 



Remarks. — ^As in the case of blandus, this mouse prefers damp, 

 gjassy places; but it appears to be more prolific. A specimen col- 

 lected at San Luis Springs (Lang's Ranch), September 3, 1893, 

 contained 8 fetuses; another, taken at the head of Babacomeri 

 Creek, Arizona, October 19, 1893, contained 6; two from the Santa 

 Cruz River at Monument No. Ill (topotypes), October 19, 1893, 

 contained 6 and 7; another from the same locahty, October 23,1893. 

 contained 7. 



Record and measurements of 36 Specimens of Peromyscus sonoriensis. 



