n PREFACE. 



specific names in the old, and sixty-six in the new Catalogue. 

 But, in comparing these two lists, we find that they have only 

 eleven names in common. In other words, of the forty-two so- 

 called species of Trypeta of the old Catalogue, only eleven are 

 adopted now as specific names in that genus ; the other thirty- , 

 one names proved , upon investigation , to be either synonyms, 

 or to represent species which had been erroneously placed in 

 the genus Trypeta, or else to be unavailable names , on account 

 of the insufficiency of the descriptions. The ditierence between 

 eleven and sixty-six (the number of species in the new Catalogue), 

 represents therefore the addition made to the knowledge of the 

 genus Trypeta in North America during the interval between the 

 two catalogues. Other genera give similar results. Thirty-two 

 species of BolieJioxms were described previous to 1858; the 

 present list contains fifty-nine; but both lists have only tico 

 specific names in common. Thirty of the earlier descriptions 

 are unrecognizable and therefore useless. The old Catalogue 

 contained 32 names of species of Eristalis, occurring in North 

 America, north of Mexico; of these names only nine figure as 

 species of Eristalis in the present Catalogue, althoigh the de- 

 finition of the genus has not been changed since then. The other 

 names of the old Catalogue are either synonyms (E. dimidiatus, 

 for instance, has been described under six different names), or 

 they belong to other genera, as Hclophilus, Milesia, even XyJota. 

 The genus Tahamts, in the old Catalogue, contains one hundred 

 and two names of species, from North America, north of Mexico ; 

 among these names only 36 could be adopted; the remainder 

 are either synonyms, or absolutely unavailable, on account of the 

 insufficiency of the descriptions. — These instances will suffice 

 to show that the new Catalogue is, not merely a new edition of 

 the old one, only supplemented by the new species, published 

 between 1858 and 1878; it is a new work, prepared on a diHe- 

 rent plan. 



The process gone through between two editions of a cata- 

 logue, (the compilatory and the critical edition), consists in form- 

 ing collections, in determining them from existing descriptions, 

 and thus making out the synonymies, and then working up each 



