242 University of California Publications in Zoology [Vol. 12 



ground of the upper mesa, and occasionally on rocky hillsides. These 

 instances probably show great extent in foraging radius, the home 

 burrows being in the softer ground of neighboring ravine bottoms or 

 in depressions where sand could lodge. For, as far as observed, the 

 burrows are always dug by the animals themselves, and their burrow- 

 ing powers are weak. 



Their small size, however, does not require anywhere near the depth 

 of workable soil that the huge D. deserti needs. Probably the far 

 greater restriction in distribution of the latter species compared with 

 that of D. merriami is determined by this difference in depth of soil 

 required. 



The burrows of D. merriami are not easy to locate, as the entrances 

 are left smoothly closed during the day. But where the fine wind-laid 

 sand composed the surface of the ground, the hind foot and tail tracks 

 showed clearly the routes taken by the rats to and from their burrows. 



The breeding season is at its height in April, though the following 

 dates show T considerable latitude. Females with embryos were taken 

 on February 19, March 5 and 6, April 1 to 4, 19 and 21, and May 8. 

 On each of the above dates but one animal is concerned, except during 

 the period from April 1 to 4, when observations were made upon at 

 least ten, and April 19, upon two. In sixteen cases there were two 

 foetuses, in two cases three. 



The above data indicate a relatively slow rate of reproduction : 

 but one litter per year is raised and there are seldom more than two 

 young to a litter. This indicates a much safer existence, individually, 

 for this kangaroo rat than for the ground squirrels and white-footed 

 mice of the same habitat. In the case of these latter rodents litters 

 consist of four to eight young, and in some species, at least, two or 

 more litters are born each year. Kangaroo rats must be relatively 

 very successful in escaping the numerous enemies that assail the rodent 

 population of the desert. 



The writer finds it impossible to refer the Colorado River series of 

 Dipodomys m< rriami, as a whole, to the subspecies D. m. simiolus (type 

 locality. Palm Springs, on the western arm of the Colorado Desert), 

 as might have been expected. The material shows great range in 

 variation; some of the specimens, particularly in the series from the 

 lower course of the river, are. it is true, very close to simiolus. On 

 the other hand many, especially from the Arizona side of the river 

 above Ehrenberg, are not with certainty distinguishable from near- 

 topotypes of D. m. merriami, from south-central Arizona. It proves. 



