THE GENERA HYDROCHUS, OCTHEBIUS, AND HYDRJENA. 159 



A. Length ^ a line, or thereabouts. 



0. margipallens, Latr. — Thorax with very feeble central 

 furrow, and transverse impressions above and below, which are 

 not continued to the sides ; membranous border distinct, more so 

 at the posterior angles ; elytra not very strongly, although 

 distinctly, punctured. Legs red. A generally distributed 

 species ; found chiefly in brackish ditches ; commoner in the 

 South of England than in the North. Hanwell, Whitstable, 

 Sheerness, Gravesend, Hunstanton, &c. 



O. exaratus, Muls. — At once distinguished by its very shining 

 appearance, deep punctuation of elytra, and by the deep sculpture 

 of the thorax, which is very transverse, and has a feeble central 

 furrow and two deep transverse furrows, which are continued 

 to the lateral margins ; this latter character is found in no 

 other British species. Posterior angles of thorax excised, and 

 filled with membrane. Legs pitch} r . Usually a very rare 

 species ; but has been found in abundance by Dr. Power (who 

 introduced it as British), and also by Mr. Champion. Rainham, 

 Lewes, Southend, Gravesend, Whitstable. It occurs in brackish 

 ditches, as a rule. 



0. Poiveri, Rye. — Thorax with central furrow and four 

 depressions, aad with posterior angles excised and filled with 

 membrane ; legs and palpi dark. Distinguished from both the 

 preceding species by the sculpture of the thorax ; and from 

 0. exaratus more particularly by its duller appearance, closer 

 punctuation, posteriorly more contracted thorax, &c. The first 

 specimen taken by Dr. Power at Seaton, Devonshire ; and several 

 near the Chesil Bank, Weymouth, by Dr. Sharp and Mr. Crotch. 



B. Length usually 1 line ; never less than f line. 



a. Thorax with very feeble central furrow, sometimes 

 marked by a depression, and two strong transverse impressions 

 not continued to the sides; membranous border distinct. 



0. marinas, Payk. — Thorax very finely punctured, almost 

 smooth, often iridescent ; elytra finely punctured, usually more 

 or less castaneous ; membranous border of thorax very indistinct. 

 In general sculpture it approaches most nearly to 0. margipallens, 

 but may at once be distinguished by its size, colour, and wider 

 thorax. A very common and generally distributed species in 

 brackish ditches or stagnant pools near the sea. I have found it 

 in great abundance at Hunstanton, Norfolk, in company with 

 0. hicolon and 0. ceratus. 



