264 
JOURNAL OF MAMMALOGY 
RECENT LITERATURE 
Grinnell, Joseph. A Geographical Study of the Kangaroo Rats op 
California. University of California Publications in Zoology, vol. 24, no. 1, 
pp. 1-124, pis. 1-7, text figures 1-24. June 17, 1922. 
Grinneirs ‘‘Geographical Study of the Kangaroo Rats of California” presented 
with a wealth of pertinent detail is not only a valuable contribution to our knowl- 
edge of an exceedingly interesting and imperfectly known group of rodents, but 
has an important bearing in a wider field as well. The main purpose of the work 
as stated by the author has been “to ascertain the degree of correlation which 
exists between speciation and geography and environment,” and perhaps no 
better group of mammals, owing to its plasticity, or a better area, due to highly 
diversified environmental conditions, could have been chosen. 
The author’s conclusions are based upon a study of 2,834 specimens from Cali- 
fornia, of which 2,212 skins with skulls, representing 213 localities, are in the 
Museum of Vertebrate Zoology, University of California . Four hundred and nine 
specimens of kangaroo rats in the collection of the Biological Survey, United 
States Department of Agriculture, representing species and localities deemed of 
special importance by the author, were shipped from Washington, D. C., and 
material was made available by other institutions. 
Thirty-three species and subspecies of kangaroo rats are recognized as ranging 
within the State of California, “as many forms as occur all told outside of this 
State and at the same time do not reach into it.” These are assigned to the Heer- 
manni, Ordii, Merriami, Agilis, Microps, and Deserti groups, the group names 
being those of typical included species. 
Until Doctor Grinnell began his work upon the kangaroo rats, three generally 
accepted genera, Dipodomys, Perodipus, and Microdipodops, were assigned to 
the subfamily Dipodomyinae. The generic separation of Dipodomys and Perodi- 
pus rested upon the supposed constancy of a differential number of complete 
digits on the hind foot. The author, however, has made the important discovery 
that in parts of the distribution area of Dipodomys heermanni the small rudimen- 
tary first toe on the hind foot may be present or absent . The detection of positive 
evidence of intergradation in this character within a limited area between forms 
obviously not otherwise materially different is of special interest, in view of the 
remarkable constancy with which a fixed number of digits (4 or 5) on this foot is 
maintained in all of the species throughout the remainder of the wide range of 
the subfamily from the Valley of Mexico to southern Washington and Montana.' 
All of the species heretofore assigned to the five-toed genus Perodipus are therefore 
transferred to the genus Dipodomys. No reason is noted for the omission of the 
genus Microdipodops, currently regarded as a true kangaroo rat and represented 
in California by one species, the inclusion of which would have completed the 
treatment of the subfamily within the state. 
' This condition seems somewhat analogous to that obtaining in certain squir- 
rels, and in bats of the genera A r/fbews and Myotis. The number of premolars con- 
stant in most of the included species is variable, owing to obsolescence in several, 
and the tooth formula therefore becomes unreliable as a generic character. 
