236 University of California PuMications in Zoology [Vol. 12 



10; so that there is not such constancy in breeding time in this species 

 as in certain other rodents of the region. 



A careful comparison of the six specimens secured from the Arizona 

 side with the much larger series from the California side, shows both 

 cranially and as regards external features, no tangible differences 

 which might be expected to occur (and do occur in other cases) in a 

 mammal of the desert proper and whose range is divided by the 

 Colorado River. 



It appears that this species of wood-rat has not previously been 

 found to the east of the Colorado River (see Goldman. 1910, pp. 76, 

 77). In fact, in his revision of the genus Neotoma, Goldman, in dis- 

 ciassion of Neotoma lepida and its subspecies stepliensi (which are 

 forms not dissimilar to desertorum) . states (1910, p. 80) that "the 

 ranges of the two [lepida and desertorum] are completely separated by 

 the effective barrier of the Colorado River." While the results of our 

 work detract from this statement as to fact, the implication remains 

 the same, namely, that the Colorado River may have been at one time 

 of prime service in effecting the isolation of the lepida stock, particu- 

 larly in the upper (northea.stern) course of the river (see Goldman's 

 map, p. 77). 



I sent my Arizona-side examples of desertorum to Mr. Goldman 

 with the request that he examine them closely and give his opinion 

 as to their possible approach in characters to N. I. stephensi. The 

 latter form is, by the way, the nearest wood-rat of the same group 

 recorded from east of the river, its nearest station being (Goldman, 

 1910, p. 80) the Hualpai Mountains, Arizona, only about fifty miles 

 east of The Needles. ]Mr. Goldman replied that my specimens were 

 "typical desertorum,'' and hence not bearing any significant resemb- 

 lance to stephensi. 



It would appear, therefore, that the desertorum, as found by us 

 on the Arizona side of the Colorado at the three stations named, is of 

 probably direct and relatively recent descent from the stock on the 

 California side. While this wood-rat does not inhabit the river bottom- 

 lands, as does Neotoma a. venusta, it is quite possible that individuals 

 forage down to the water's edge where the riparian strip is reduced 

 in width or practically wanting, as at The Needles. In event of rats 

 becoming marooned on drift-rafts at periods of rising water such 

 animals would be in a position to be ferried across the river; for it is 

 not unlikely that such rafts would be carried by the swerving currents 

 to opposite banks. No matter where such waifs should be landed they 



