850 On Aquatic Carnivorous Coleoptera or Dytiscidoe. 



conceals the articular cavity : the ridge on the inner face of the wing-case is broader 

 near the apex and distinctly bisinuate. (So far as 1 can see there is only one verv 

 short claw terminating the hind tarsus, but this requires confirmation.) 



When fully known I think it probable the departure from Hydrovatus will be 

 found very decided ; it is worthy of note that this interesting form approaches 

 considerably in appearance and in some of its characters to its geographical consort, 

 Pachydrus. 



This autogeneric species is found in tropical America. 



I. 19.— Genus HETERHYDRUS. {Vide p. 337.) 



This autogenus is formed by a species whose individuals have the size and form 

 of an elongate Hyphydrus, and this resemblance is rendered still greater by a 

 similarity in the head and swimming legs to those of Hyphydrus. The head is quite 

 rounded and evenly margined in front, and the labrum is exserted on the under- 

 surface; the prosternal process behind the coxae is very broad and short, and its 

 hind margin forms in the middle so very obtuse an angle that it may be called sub- 

 truncate. The hind coxte are extremely large, and the suture between them and 

 the metasternum is very obliterated; their articular cavities.are widely separated, 

 and quite exposed, being placed external to the broad adpressed coxal process, and 

 not protected by any portion of it. The swimming legs are apparently similar to 

 those of Hyphydrus, even as regards the tarsi and claws. As I have only a single 

 (badly preserved) individual of this creature at my disposal I am unable to ascertain 

 more completely its characters. It appears however to be allied to the South 

 American Pachydrus, from which it differs by the exserted labrum, and by the fact 

 that its hind coxae have their anterior border less arched, and less approximate to 

 the middle coxae. Whether the coxae be soldered with the ventral segments is not 

 ascertained, but in my classification I have taken it for granted that such is the 

 case. 



This interesting autogenus is found in Madagascar and tropical Africa; 1 have 

 little doubt that other species will be discovered. 



I. 20.— Genus PACHYDRUS. {Vide p. 338.) 



Five species form this aggregate ; they are insects of broad and short, rounded 

 form, excessively convex beneath, and without colour variegation on the surface. 



The head is broad and short, and is rounded and margined in front, the labrum is 

 not visible from the front, but when the undersurface of the head is looked at it is 

 found to be slightly exserted. The prosternum is very small, and has a short, 

 broad — broader than long — process ; this prosternal process is carried forwards 

 between the coxae as a raised narrower process, till it terminates very abruptly, 

 more than lialf way towards the front of the coxae, as an abrupt acute projection. 



