PREFACE. XIX 



applies to Mr. Walker's descriptions in the genera Clirysops and 

 Tabanus. A careful study of these descriptions convinced me 

 of their uselessness; the examination of Mr. Walker's types 

 showed, that in most cases, he did not know his own species 

 again, that he described the same species several times in 

 succession (the descriptions being sometimes by the side of each 

 other in his works), that the confused specimens of different 

 species in the same description. Under such circumstances, I 

 did not feel justified in upsetting the nomenclature introduced 

 by me in my monograph. 



The authorities of the British Museum, in a most praise- 

 worthy, and truly scientific spirit, have bestowed a great deal 

 of labor upon preserving and labelling Mr. Walker's types. 

 But the task of singling out the original type of the description, 

 from among the specimens added afterwards, is by no means 

 an easy one, often hardly possible. Furthermore, it is a well- 

 known fact that authors are apt not to be very careful with 

 their own types; to remove and diplace them, when made 

 aware of an error; and Mr. Walker, in this respect, was not 

 an exception. Neither his, nor any other types can, therefore 

 be implicitly relied upon, and we have, ultimately, to fall back 

 on the descriptions. In rescuing those of Mr. Walker's de- 

 scriptions, which are available and in rejecting the remainder, as 

 useless , we pursue , I think , a course consistent both with 

 justice and scientific expediency. 



THE NUMBER OF DESCRIBED NORTH AMERICAN DIPTERA. The 



number of described Coleoptera from North America, north of 

 Mexico, in Mr. Crotch's Check List is 7450. It is impossible 

 to make a similar statement for the diptera, because, as ex- 

 perience has shown, most of the earlier descriptions are entirely 

 unavailable and represent species which exist merely on paper. 

 The number of described diptera from North America, north of 

 Mexico, contained in the Museum of Comparative Zoology in 

 Cambridge, Mass., is a little over 2000. The . number of 

 available, but not yet identified, descriptions of earlier authors 

 is not large ; and thus we may safely assume that, excluding the 



