HERCbSTOMUS. 117 



The characters of the genus may be defined as follows : An- 

 tenna? of ordinary structure ; the first joint hairy on the upper 

 side ; the second joint of the antennas transverse ; the third 

 joint not elongated ; arista dorsal, of the usual plain structure. 

 Scutellurn without hairs. Face not reaching as far as the in- 

 ferior orbit. Hypopygium on a very short peduncle, so as to 

 appear sessile ; exterior appendages lamelliform ; interior append- 

 ages but little developed. The first joint of the hind tarsi without 

 bristles. The first posterior cell narrowed towards its end ; tlje 

 last segment of the fourth longitudinal vein only gradually ap- 

 proaches the third longitudinal vein. 



The differences from the genera Gymnopternus, with its third 

 and fourth longitudinal veins parallel, from Pelastoneurus with 

 its feathered arista and the fourth longitudinal vein strongly in- 

 flected forwards, Paraclius with the end of the fourth longitu- 

 dinal vein angularly inflected forwards and then running towards 

 the margin of the wing, in the shape of a segment of a circle, are 

 self-evident. The species of Hercostomus differ from those of 

 Hypophyllus by their arista, whieh is plain in both sexes, by the 

 apparently sessile hypopygiuin and by the lesser development of 

 its interior appendages. 



Up to the present time only European species have been made 

 known ; I am now enabled to add to them a North American 

 species. 



The name of the genus (from jpxoj, wall, fence, and otopa, 

 mouth) has reference to the oral opening, surrounded, fence-like, 

 by the suctorial surface covered with rows of hairs ; this being the 

 case with those species on which I had originally established this 

 genus. 



1. H. ii til color, n. sp. . Obscure viridis, nitidus, antennis, ocu- 

 lorum tegularumque ciliis pedibusque totis nigris, alis ciuereis, lamellis 

 hypopygii ovatis, nigricantibus, in disco sordidissime ex albidis. 



Dark-green, bright ; antennae, cilia of the inferior orbit and of the tegulas, 

 also the feet, black ; wings gray, lamellae of the hypopygium oval, 

 blackish, upon their middle very dingy whitish. Long. corp. 0.11 

 0.12. Long. al. 0.13. 



Dark metallic-green, almost black-green, bright. Front me- 

 tallic-green. Antennae black ; third joint oval, at the tip only with 

 a blunt point. The color of the narrow face seems to have been 



