Illjdropdni^.'] ADEPHAOA. 173 



it is acknowlcdgod by all students of the Dytiscidre to be in many cases 

 a very valuable character ; it is, however, in most cases very evident, as 

 will be plain to any one who will compare together //. dorsalis or lineafiis 

 with H. j/^amis or nigriia, and the species concerning which any 

 difficulty might arise as to which section they belonged (as //. memnonhit!, 

 ohsoletus, and ferruginew^) are, as a rule, easily distinguished by other 

 characters, and are mostly rare and seldom met with. 



The colour of the species is in many cases a great help towards dis- 

 tinguishing them, but it is not possible to classify them by this, as it is 

 in the case of I5embidium, as the colour varies very considerably, and 

 some of the species that as a rule show markings at base of elytra or on 

 their disc occasionally have them wanting: the succeeding table, how- 

 ever, may be of some service, as long as it is remembered that it is 

 merely provisional and applies to mature specimens only. 



1. Elytra black with yellow lines or markings distributed over disc : H. pictus, 

 granulnris, Jlaripei; (very small species), H. lepidu.i, palustris, incognitus (medium- 

 sized species), H. dorsalis (larger species, markings sometimes visible at base onlv, 

 sometimes almost absent). 



2. Elytra testaceous with black markings : H. halensis, Davisii, xeptenfrionalis 

 riv'ilix. 



'3. Elytra black with lipfliter markings apparent at base only, or close to mar- 

 gins : JI. firi^throcepkalus, planus, rujifrons, puhescpiis, jv7/!( /a (lighter markings 

 usually more or less obsolete, sometimes hardly traceable), M. marginalus, IHuratus 

 (lighter markings always plain). 



4. Elytra ferruginous or olive brown suifused with a lighter colour, especially at 

 base (species long oblong, all very rare) : H. oblovgns, Jerrugineus, ohsoletus. 



5. Upper surface unicolorous black or pitchy brown (liead sometimes lighter and 

 thorax darker) : U. discrefus, nigi-'da, memnonius, me/anarius and v. nionlicola 

 longicornis, Jongulus {cela/us), morio (alriceps), Gi/Uenhalii, umbrosus, neglectus 

 tristis (the latter three are occasionally a little lighter at base and shoulders). 



6. Upper side retldish, head and, as a rule, thorax lighter than elvtia : IT. 

 angustatus, obsctirus, /Scales ia nil s (unicolorous), JI. lineatus (with dark' lines ou 

 elytra). 



1. Thorax with a small longitudinal impression on each 

 side at base, 

 i. Antennaj with fourth joint plainly smaller thnn 

 the joints above and below it ; sjiecies very snutll, 

 black with yellow markings on disc of elytra. 



1. Uody oval, convex ; head red II. I'lcn s, T''. 



2. Hody oblong, rather depressed ; head black. 



A. Elytra with side border and two longitudinal 



lines (one dorsal) yellow H. Guanclauis, L. 



B. Elytra with side border and four lines (often 



abbreviated and interrupted) yellow .... II. fj.avii'ks, Ol. 

 ii. Anttnufe with fourth joint equal to the joints 

 above and below it ; species larger (2;]- 1 mm.). 



1. Head bhick ; elytra black with yellow markings II. LEl'IUUS, OL 



2. Head entirely or almost entinly testaceous; 



elytra testaceous with black markings. 



A. Form short oval ; thorax with a more or less 

 distinct dark baud ou disc ; elytra almost im- 



puuctate II. mvALis, Ggll. 



B. Komi long oval; elytra with distinct scattered 

 j)unclures. 



