160 THE ENTOMOLOGIST. 



b. Thorax with well-marked central furrow, but no impression 

 on the disc ; membranous border distinct at posterior angles. 



0. pygmceus, Fab. — Distinguished from the next species by 

 the much stronger punctuation of its elytra, which are less dilated, 

 and by its duller appearance ; the thorax also is much more 

 strongly punctured, and the central furrow is more marked. The 

 commonest species of the genus. Found in fresh water (stagnant 

 ponds and ditches) throughout the kingdom. 



0. eerie us, Steph. — Brassy testaceous, very shining, with elytra 

 considerably dilated. Distinguished from the preceding by its 

 weak striation and larger, though feebler, punctuation ; and from 

 all other species by the sculpture of its thorax. A very rare 

 species; but has been taken by Dr. Power at Birdbrook (Essex), 

 at Cowley (near Uxbridge), at Hanwell, and at Horsell. 



c. Thorax with central furrow and two well-marked impres- 

 sions (forming a semicolon) on each side ; posterior angles 

 deeply excised, and filled with white membrane. 



O. bicolon, Germ. — Thorax transverse ; elytra with strongly 

 punctured striae. Insect of an entirely dull brassy colour. 

 Generally distributed, and common in stagnant brackish water. 



0. rufimarginatus, Steph. — Allied to the preceding species, of 

 which it is considered a variety in De Marseul and Gemminger 

 and Von Harold's catalogues. Its short thick form and more 

 ovate elytra, and the broadly rufous margins of its thorax, are, 

 however, abundantly sufficient to give it specific value. My 

 specimens are smaller than O. bicolon. Scarce, but generally 

 distributed ; and found both in fresh and brackish water. 

 Repton, Birdbrook, Lee (Kent), Sheerness, Tottenham, Scar- 

 borough, &c. 



0. ceratus, Steph. — Distinguished from the preceding species 

 by its dull, almost black, colour, and by its smooth elytra, which 

 are very finely punctured, with scarcely visible striae. It is just 

 possible that small specimens of this species might be at first 

 sight confounded with the largest 0. margipallens, but the 

 sculpture of the thorax will at once separate them. Local, and 

 not very common ; found in brackish ditches and ponds. 

 Whitstable, Sheerness, Southend, Deal, Gravesend, Hunstanton. 



C. Length, lg- line. 



0. punctatus, Steph. — This species comes in naturally after 

 the three preceding, which it resembles in the sculpture of the 



