SUMNER — DESERT AND LAVA-DWELLING MICE 
81 
‘‘Specimens taken in lava beds are usually darker than those inhabit- 
ing hghter-colored rock formations.’^ Similar statements are made in 
respect to other species on pages 81 and 102. Unfortunately, we have 
no record of the number of individuals on which these statements are 
based, save that in the case of N. lepida stephensi but a single speci- 
men was recorded from the lava beds. We likewise have no informa- 
tion as to the altitude, meteorological conditions, etc., of the particular 
lava fields where the species in question were trapped. 
The chief direct evidence which I have to offer on the present subject 
was obtained during a collecting trip undertaken in the spring of 1920. 
The choice of locality was due primarily to the suggestions of Prof. 
Joseph Grinnell, director of the University of California Museum of 
Vertebrate Zoology. Doctor Grinnell, succeeded by Mr. Richard Hunt, 
of the same museum, together with the writer, constituted the field 
party. 
The lava field on which the mice were trapped lies in the Mojave 
Desert some 12 miles west of the village of Ludlow, and just south of 
the main line of the Santa Fe Railway. Unfortunately, no Geological 
Survey or other reliable map of this region exists. The outline of the 
field is very irregular, the greatest length being perhaps five miles and 
the greatest width three. 
Regarding the age of this eruption I can learn nothing definite. 
The lava, throughout much of the area, looks extremely fresh, and a 
beautifully preserved cinder-cone (“Mt. Pisgah”) occurs near the 
northern border. On the other hand, there are, so far as I know, no 
hot springs, fumaroles or other evidences of recent volcanic activity 
in this part of the desert. Whether the age of the field is to be reck- 
oned in hundreds or in thousands of years I am unable to learn from 
the geologists whom I have consulted. 
The surface of this lava bed is raised well above the general level of 
the desert. It is extremely rugged and difficult of passage, being 
crossed in every direction by jagged ridges and yawning fissures. Sand 
has drifted in from the surrounding desert and become deposited in 
cracks and depressions, affording soil for the support of scattered shrubs 
and annuals, even a mile or more from the nearest border. Despite 
the presence of occasional sand pockets of considerable extent, the 
prevailing tone of the lava field is extremely dark. Viewed from 
neighboring hilltops it everywhere stands in extreme contrast to the 
pale sand and gravel which surrounds it on every side. Furthermore, 
it must be emphasized that the mice are not confined to the small 
