52 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



In the preparation of the foregoing paper my best thanks are due 

 to Mr. G. S. Miller, curator of the Division of Mammals in the 

 United States National Museum, for the free use of the material 

 under his charge, as well as for much kindly help and advice. I am 

 also especially indebted to Messrs. J. W. Gidley and Charles W. 

 Gilmore, of the Division of Vertebrate Paleontology of the Museum, 

 for much valuable help and assistance. In the course of prepara- 

 tion of the manuscript I have discussed with these keen students of 

 the subject each and every point of the conclusions herein reached, 

 and I feel that they are entitled to something more than a mere 

 formal acknowledgment. The illustrations have been faithfully 

 executed by that veteran illustrator, Rudolph Weber, in his usual 

 masterful manner. 



