304 DIPTERA OP NORTH AMERICA. [PART IV. 



to that of Cyl. nodicornis 0. S. The ovipositor of the female 

 has short, broad, obtuse valves. 



The principal feature of the venation, the course of the first 

 longitudinal vein, which does not end in the costa, but is incurved 

 at the tip towards the second vein, is the same here as in Cylin- 

 drotoma. The auxiliary vein ends abruptly at the inner end of 

 the stigma, and has but an indistinct connection with the first 

 longitudinal vein (I perceive this in T. exsculpta only). A 

 slight vestige of an oblique cross-vein connects the latter part 

 of the first vein with the costa. The absence of the small cross- 

 vein seems to be a peculiarity of this genus ; the inner end of the 

 submarginal cell thus comes in immediate contact with the discal 

 cell. 



The relationship of Triogma and Cylindrotoma is very great, 

 and the principal differences consist in the structure of the an- 

 tennae, the number of posterior cells, and the absence of the small 

 cross-vein in the former genus ; in the shape and sculpture of 

 head and thorax, and in the general coloring. 



This genus was proposed by Dr. Schiner, in 1863, for the 

 European Limnobia trisulcata Schum. ; the North American 

 species is an exactly analogous form. These two species are the 

 only ones of the genus at present known. 



The name of the genus is derived from tpsis, three, and 6ynb$, 

 furrow, in allusion to the thoracic furrows/ 



Description of the species. 



1. T. exsculpta 0. S. 9 • — Fusca, alis infuscatis ; thorace sulco 

 medio impresso ; vittis ejus lateralibus, capite metanotique parte an- 

 teriori rugoso-punctatis. 



Brown, wings tinged with brownish. ; thorax with an impressed groove in 

 the middle ; its lateral stripes, the anterior part of the metathorax, and 

 the head are rugoso-punctate. Long. corp. 0.37. 



Syn. Triogma exsculpta 0. Sacken, Proc. Entom. Soc. Phil. 1865, p. 239. 



Whole body dull brown ; front and vertex rugoso-punctate, 

 with a longitudinal furrow in the middle ; palpi brown ; antennae, 

 basal joints brown, flagellum paler, joints of the latter subovate. 

 Thorax with a deep longitudinal furrow in the middle ; the lateral 

 stripes are marked by a deep, irregular punctation ; from the 

 anterior part of these rugoso-punctate stripes a similar punctation 



