HYDROPIIILIDiE. 45 



Tribe I.— If ELOPIIORIXI. 



In this tribe are small aquatic species, of an oblong or elongate 

 form, usually of a pale gray color, more or less tinged with bronze 

 or silver. They are found in small pools, and rise to the surface 

 when the water is made turbid. 



Maxilla? with both lobes corneous ; antenna? 9-jointed, rarely 

 7-jointed. Tarsi not natatorial ; first joint subconnate with the 

 second, frequently indistinct ; 2 — 4 moderate, subequal, the second 

 in Helophorus somewhat longer than the first. Thorax narrower 

 at the base than the elytra ; in Helophorus and Ochthebius 

 marked with five sinuous longitudinal stria? ; elytra with ten stria? 

 or rows of punctures, except in Hydra?na, where the rows are 

 more numerous. 



But one genus (Epimetopus, having the eyes emarginate) of 

 this tribe has not occurred in the United states ; the others are 

 thus related : — 



Last joint of maxillary palpi longer than the preceding ; 



Antenna? 9-jointed ; all the palpi moderately long. Helophorus. 



Antenna? 7-jointed ; labial palpi short. Hydkochus. 



Last joint of maxillary palpi shorter than the preceding, subulate. 



Ochthebius. 

 Maxillary palpi exceedingly long. Hydb^na. 



The last genus has occurred as yet only in the Atlantic States ; 

 the other three are found generally distributed over our territories. 



Tribe II.— HYDROPIIILIOT. 



Aquatic species, of an oval or elliptical convex form, olive 

 black, rarely with the sides of the thorax and elytra yellow, the 

 latter not striate. 



Maxilla? with both lobes coriaceous ; antenna? 9-jointed ; mid- 

 dle and posterior tarsi strongly compressed, fringed internally 

 with long hairs ; first joint short, second elongated ; meso- and 

 metastcrnum forming a continuous keel, which posteriorly is pro- 

 longed into an acute spine ; last joint of the anterior tarsi of the 

 male in many species distoi'ted, with very unequal claws ; in the 

 same sex the club of the antenna? is sometimes irregular. 



