ADVERTISEMENT. 



THE present work was undertaken by Baron C. R. Osten 

 Sacken, of Russia, as a revision and extension of a Catalogue 

 of Diptera prepared by him twenty years ago, and published by 

 the Smithsonian Institution in Volume III. of its Miscellaneous 

 Collections. It is, however, not merely a new edition of the 

 volume in question, but an entirely new work, constituting a 

 valuable contribution to our knowledge of the entomology of 

 North America. 



SPENCER F. BAIRD, 

 Secretary Smithsonian Institution. 



WASHINGTON, October, 1878. 



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