98 DIPTERA OF NORTH AMERICA. TPART II. 



I defined the characters of this genus and its differences from the 

 neighboring genera. America possesses species which necessarily 

 come within the definition of the genus Gymnopternus, as under- 

 stood in that publication, but which, at the same time, differ too 

 much from all other species of this genus, to find a natural place 

 among them. The most striking, although perhaps not the most 

 important, character whereby these species differ from the others, 

 is the course of the last segment of the fourth longitudinal vein. 

 At or beyond its middle it is suddenly deflected anteriorly, and its 

 end is so near the end of the third longitudinal vein that the first 

 posterior cell appears almost closed. In order to separate these 

 species from the genus Gymnopternus, I have added above to the 

 characters of Gymnopternus the complete, or at least nearly com- 

 plete, parallelism of the third and fourth longitudinal veins. A 

 more minute examination of the species in question shows that they 

 should form two, or perhaps more correctly, three groups ; still, 

 before we are able to judge with certainty about it, our as yet im- 

 perfect knowledge of the species will require a considerable in- 

 crease. In the meantime, however, if we draw our attention to 

 the character which distinguishes all these species from the other 

 Gymnopternus, that is, to the course of the last segment of the 

 fourth longitudinal vein, we will soon find among these species 

 two principal modifications of this course. In one case the deflec- 

 tion of the fourth segment at or beyond its middle takes place in 

 a steep curve forward, and the vein then runs in a straight direc- 

 tion to the margin of the wing, which it reaches very near the tip 

 of the third longitudinal vein. In the other case the last segment 

 of the fourth longitudinal vein forms beyond its middle a but 

 slightly rounded angle, and thence, in the form of a curve, the 

 concavity of which is turned backwards, it runs to the margin of 

 the wing, which it likewise reaches in the immediate neighborhood 

 of the third longitudinal vein. Those species which show the first 

 of the above mentioned neurations, possess, moreover, many other 

 characters in common, which distinguish them from the species of 

 (i ymnopternus, and thus they form the genus Pelastoneur%s. As 

 the most important of these characters may be mentioned the 

 feathered arista, the broad face, which is common to both sexes, 

 strongly convex upon its lower part, and provided with a sharp, 

 curved inferior margin ; also the elongated and distinctly pcduncu- 

 Jated hypopygium. On the contrary all those species, in which the 



