850 On Aquatic Carnivorous Coleoptera or Dytiscide. 
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 very 
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 1m 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 coxze 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 coxze 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 coxee have their anterior border less arched, and less approximate to 
the middle coxe. Whether the coxz 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; I have 
little doubt that other species will be discovered. 
I. 20.—Genus PACH YDRUS. (Vide p. 338.) 
Five species form this aggregate ; they are insects of brgad 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 coxee as a raised narrower process, till it terminates very abruptly, 
more than half way towards the front of the coxs, as an abrupt acute projection. 
