1865.] 23!) 



four have a small cross-vein. The three known species of Triogma 

 have only four posterior cells. 



The relationship of Triogma and Cylindrotoma is very great, and 

 the principal differences consist in the structure of the antennae and 

 the frequent absence of the small cross-vein in the former genus. The 

 general appearance of Triogma is heavier than that of Cylindrotoma, 

 the feet are stouter, the abdomen loss .slender at the basis, but also (in 

 the male) less club-shaped at the tip; the first antennal joint of C. 

 americana is not much longer than the second, whereas it is distinctly 

 longer in Triogma. 



This genus was proposed by Dr. Schiuer in 1863 for the European 

 Limnobia trisulcata Schmnmel; a precisely analogous form is the North 

 American Triogma exsculpta. T. nodicornis is somewhat more like 

 Cylindrotoma in its coloring and its general appearance; still the sculp- 

 ture of its thorax and the frequent absence of the small cross-vein on 

 its wings apparently refer it here, uuless, on account of the peculiar 

 structure of its antennae, we establish a separate genus for it ; this, ac- 

 cording to my opinion, would be altogether useless. These three spe- 

 cies are the only ones of the genus at present known. 



The name of the genus is derived from tpiis, three, and 67,1165, furrow, 

 in allusion to the thoracic furrows. 



Description of the Species. 



1. T. exsculpta, n. sp. 9 . — Fusca, alis infuscatis : thoraee sulco medio impresso : 

 vittis ejus lateralibus, capite metanotique parte anteriore rugoso-pimctatis. 



Brown, wings tinged with brownish: thorax with an impressed groove in the 

 middle; head, lateral stripes, and the anterior part of the metanotum, rugoso- 

 punctate. 



Long. corp. 0.37. 



Whole body dull brown ; front and vertex rugoso-punetate, with a longitudi- 

 nal furrow in the middle; palpi brown; antennae, basal joints brown, flagellum 

 paler, joints of the latter sub-ovate. 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 rugo o-punctate stripes a similar punctation ex- 

 tends backwards, along the sides of the mesonotum ; the anterior part of-the 

 metanotum is deeply rugoso-punetate, and some parts of the pleurae are also 

 punctate. Halteres dingy brownish-tawny ; coxae brown, feet tawny, clothed 

 with black hairs. Abdomen brown. An indistinct cross-vein connects the latter 

 part of the first longitudinal vein with the costa; the basal part of the submar- 

 ginal cell is in contact with the discal cell, so that there is no room for a small 

 cross-vein; the discal cell is elongated, quadrangular. 



Hab. — Pennsylvania (Cressou); a single female specimen. 



2. T. nodicornis, n. sp. % 9 • — Obscure flava, capite nigro, thoraee nigro-vit- 

 tato, antennis moniliformibus, articulis earum brevibus, subcordiformibus. 



