ADVERTISEMENT. 



THE following memoir was referred for examination to Dr. John Torrey and Dr. 

 1 . A. 1'. Barnard, of Columbia College, New York. They recommended its pub- 

 lication provided certain changes were made in the manuscript. These having 

 l TII made by the author, the work is published as a part of the scries of " Smith- 

 sonian Contributions to Knowledge." 



JOSEPH HENRY, 



Secretary, S. I. 

 WASHINGTON, October, 1872. 



(iii) 



