44 DIPTERA OF NORTH AMERICA. [PART III. 



ward, the fourth vein slightly forward, so that the first posterior 

 cell, very broad in the middle, is rather narrow at the end ; the 

 small crossvein is beyond the middle of the discal cell ; the two 

 posterior basal cells are of a rather considerable and equal length ; 

 the posterior angle of the anal cell is rounded. The extensive 

 picture of the wings forms, in the vicinity of the apex, more or 

 less regular crossbands. 



The genus AMPHICNEPHES, which I have established for a 

 North American species, will be characterized in the sequel. It 

 is somewhat like Platystoma, but distinguished by the not 

 swollen occiput, the flat scutellum, provided with only four 

 bristles, the broad wings and the striking divergency of the 

 longitudinal veins. 



A pretty Ortalida from Cuba, which cannot conveniently be 

 placed in any of the existing genera, gave occasion for the 

 establishment of the genus HIMEROESSA, which I will character- 

 ize below among the other North American genera. It is dis- 

 tinguished by the narrowness of the marginal and submarginal 

 cells; moreover, the posterior crossvein is prolonged inside of 

 the first posterior cell. 



Ortalis syngenesise Linn, is the type of a very well justified 

 genus, existing in Europe, Africa, Asia, and America, which 

 Rob. Desvoidy called RIVELLIA. Although the name is not 

 particularly well chosen, the objections against it are not serious 

 enough for its rejection. Besides the species described by Rob. 

 Desvoidy, the following belong to the genus Rivellia : Trypeta 

 basilaris Wied., Dacus succinctus Wied., Geroxys quadrifa- 

 sciatoMacq., Ortalis Ortoeda Walk., Tephritis melliginis Fitch., 

 and several others. Most of them agree quite well with the spe- 

 cies placed in the genus Rivellia by Rob. Desvoidy ; others, how- 

 ever, show a very gradual transition towards allied forms, which 

 cannot very well be united in the same genus with the typical 

 Rwelliae. Thus Macquart has established for one of them the 

 genus Epidesma. The transitions, however, are so gradual, that 

 it is not very easy to decide upon the best boundary for the 

 genus Rivellia. R. viridulans R. Desv., and all the North 

 American species which I know of, agree in their generic char- 

 acters with Rivellia syngenesise completely ; the same is the case 

 with R. basilaris Wied., and with several Rivellise, from the 

 southeastern region of Asia, which I possess in my collection ; 



