356 University of California Puhlications in Zoology [Vol. 21 



INTRODUCTION 



Changes taking place during postnatal growth in the bones of the 

 rodent skull seem to have received little attention either from the 

 systematist, who is primarily concerned with adult specimens, or from 

 the embryologist, who has seldom continued his observation on the 

 development of the skull beyond embryonic stages. In examining 

 various rodents it has become evident to the writer that certain bones 

 in the skulls of different genera differ widely as regards the relative 

 amount of growth attained and the age at which this growth and 

 the changes dependent thereon take place. It is the main purpose of 

 the present paper to describe the postnatal changes that occur with 

 growth in the bones of the skull of one particular rodent, the California 

 Ground Squirrel, Otospermophilus grammunis beecheyi, of which satis- 

 factory material happens to be available. Descriptions and figures of 

 some embryonic conditions are also given. Upon comparison of these 

 data with similar data for other forms it is hoped that generalizations 

 of some phylogenetic as well as of ontogenetic significance may be 

 arrived at. 



Acknowledgments 



The author is indebted to Mr. C. D. Bunker, of the Museum of 

 Natural History of the University of Kansas, and Mr. Donald R. 

 Dickey, of Pasadena, California, for the loan of specimens to supple- 

 ment those in the California Museum of Vertebrate Zoology ; to Dr. 

 C. A. Kofoid, of the Department of Zoology, University of California, 

 for the use of a series of embryos ; to Dr. Joseph Grinnell, Director of 

 the Museum of Vertebrate Zoology, for the use of the collections in his 

 charge and for guidance and detailed assistance while the work was 

 under way. Dr. C. L. Camp has read the final manuscript critically. 

 Figures 1 to 27 were made by my wife, Mary H. Hall, to whom 

 acknowledgments are due as well as for much other assistance rendered 

 in the preparation of this paper. 



