4 LETTER OF TRANSMITTAL. 



While the material upon which the present statements and discus- 

 sions are based is in many respects very incomplete, it is nevertheless 

 extremely suggestive and instructive, and a knowledge of it is sure to 

 awaken a strong interest in future labors in the same field that has been 

 irnitful of these results. 



A majority of the illustrations accompanying this article have been 

 drawn directly from specimens in the United States National Museum, 

 permission having been kindly given to do so by Professor Spencer F. 

 Baird, the Director of the museum. A large part of the specimens thus 

 used are the original types of the species which are illustrated. 



The drawings have been made with pen and ink by Dr. J. C. McCon- 

 uell, of Washington, and they have been reproduced by the photo- 

 engraving process. 



Very respectfully, 



C. A. WHITE. 



Hon. John W. Powell, 



Director of the United States Geological Survey. 



