PREFACE vii 



The author desires to acknowledge the help of many colleagues who 

 have generously responded to requests for information and assistance, 

 and, particularly, the advice and suggestions received from those asso- 

 ciated with him in the zoological department at the University of 

 Nebraska— D. D. Whitney, I. H. Blake, H. W. Manter, Otis Wade, 

 E. F. Powell, H. E. Low, and G. E. Hudson. In the preparation of 

 the manuscript he has profited by the intelligent cooperation of his 

 assistant, Elmer Palmatier. 



The Author. 

 Lincoln, Nebraska, 

 August, 1933. 



