246 HYDROPHiLiDiE. [Octhebius. 



preceding species by its much darker and less metallic colour, and the much stronger 

 sculpture of the elytra. 



O. bicolon, Germ, .{impressicollis. Cast.). Upper side bronze or 

 bronze-brown, shiny ; thorax very transverse, very strongly punctured 

 with sides contracted beliind, posterior angles excised and filled with 

 membrane, disc marked with a central furrow and two strong impressions 

 on each side, the anterior circular, the posterior oblique or almost semi- 

 circular, forming together a semicolon ; elytra with strongly punctured 

 strife ; legs reddish testaceous, tarsi darker. L. .If mm. 



Ponds and ditches, inland, and near the coast, usually the latter ; rather common 

 and widely distributed in England and Wales; Scotland, local. Lowlands, Forth and 

 Solway districts. 



O. rufimarg-inatus, Stepli. Closely allied to the preceding species, 

 of which it is considered by most authors to be merely a variety ; it is 

 distinguished by- its shorter form, the elytra being considerably shorter 

 and thicker in proportion than in 0. bicolon, duller appearance, and the 

 broadly reddish margins of thorax. L. lj-l|^ mm. 



Rare ; stagnant ponds and ditches both fresh and brackish ; Sheerness, Maid- 

 stone, Birdbrook, Lee, Tottenham; Hastings; Scarborough; Northumberland, The 

 Wansbeck ; I have taken it at Repton, Burtoii-ou-Trent, in flood rubbish, unaccom- 

 panied by the previous species. Scotland, local, Lowlands, Forth and Solway districts. 



O. nanus, Steph. (reratus, Steph.). Oblong-ovate, dark-bronze, 

 almost black ; head large, eyes very prominent; thorax transverse, but 

 rather longer than in some of the preceding species, sides excised from a 

 little behind middle and filled Avith clear white membrane ; elytra with 

 rows of rather weak punctures set in very feeble, almost obsolete, stride ; 

 apex of elytra almost truncate ; legs reddish, knees darker : the duller 

 ' and less metallic colour and smoother elytra will at once distinguish this 

 from the preceding species. L. H-lf mm. 



Brackish ponds and ditches ; also in fresh stagnant water; local, but not uncommon 

 in the Midlands and south of England, but not recorded from the extreme north or 

 from Scotlaud. Whitstable ; Sheerness; Ramsgate; Deal; Hastings; Shoreham 

 Brighton; Hunstanton; Southport ; Cambridgeshire and Hun tiugdoushiro Fens. 



O. Poweri, Eye. Elongate-ovate, dull, pitchy-black ; thorax trans 

 verse, strongly dilated in front and contracted behind, coarsely punctured, 

 anterior angles somewhat prominent, posterior angles excised and tilled 

 with membrane, disc with a central furrow and two impressions on each 

 side ; elytra with strongly punctured stride ; legs reddish, knees and tarsi 

 dark ; the palpi also are pitchy. L. 1 mm. 



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

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



The small size of this species prevents its being confounded with any 

 other except 0. exaratus, and small specimens of 0. nian/ijjallefift, from 

 both of which the sculpture of the thorax at once distinguishes it. 



