THE CANADIAN ENTOMOLOGIST. 183 



where they may easily be seen and taken, since they swim very poorly, 

 and are unable to dive rapidly. The species reported from Old Canada 

 are eight in number, one of which (obscurus) may be erroneously 

 identified, having been originally described from the Colorado River. 

 The differential characters are : — 

 A. Size larger (.23 in.), head and thorax shining, sparsely punctulate, 

 the latter sparsely granulate at sides, median sulcus slightly undu- 

 late obloiigus, Lee. 



A A. Size less (.13-. 18 in.). 



b. Elytra tuberculate (. 13 in."^ Uiberculatus, Gyll. 



bb. Elytra not tuberculate, alternate interspaces higher (.13 



in.) inqtiwatiis, Mann. 



bbb. Elytral interspaces not tuberculate nor alternating. 

 c. Hind angles of thorax obtuse. 



d. Sides of thorax rounded in front, nearly straight behind 



(. 1 8 in.) lacustris, Lee, 



dd. Sides of thorax regularly rounded. 



Legs testaceous, median thoracic sulcus undulate (.17 



in.) obscurus, Lee. 



Legs piceous, tibiae and tarsi testaceous, median thoracic 



sulcus nearly straight (.12 in.) nitiduius, Lee. 



cc. Hind angles of thorax rectangular. 



Thorax not narrowed at base, sides nearly straight, elytra 



clouded with fuscous (.12 in.) linearis, Lee. 



Thorax slightly narrowed at base, sides subsinuate, elytra 

 with fuscous markings, of which an inverted post-median 

 sutural V and two spots each side are most obvious 



(.10-15 '•''•) lineatus, Say. 



Hydrochus, Leach. 



Four species which are found in the same situations as Helophorus 



belong here, and, while differing considerably in facies from that genus, 



would at once be recognized as allied to it. The thorax is much 



narrower than the elytra, and the breadth only about equal to, or very 



slightly exceeding, the length. 



A. Smaller (.10 in.), elytral interstices not distinctly alternating, nor 



interrupted; thorax scarcely narrowed behind, sides straight, 



scarcely crenulate, basal foveae deep simplex, Lee. 



AA. Larger (.13-15 in.), elytral interstices alternating, and more or less 

 interrupted. 



