216 On Aquatie Carnivorous Coleoptera or Dytiscide. 
The side of the pronotum is most frequently limited above by a raised margin ; 
this margin may be broad as in Hyderodes, or very fine as in Eretes, or may 
be entirely absent, as in the Cybistrini and numerous others, and all the grades of 
connection exist between these extreme instances. Ina few cases the anterior edge 
of the pronotum is also finely margined as in Coptotomus and Lancetes, but this is. 
rare, and a raised front margin is generally nearly absent, although usually more 
or less distinct traces of its existence may be seen in the part of the thorax near 
to the anterior angle. In Pelobius the anterior margin is conspicuously ciliate, 
and in Amphizoa it is furnished with a narrow strip of pale membrane; in the 
other Dytiscidee it is destitute of either membrane or ciliz, except that frequently 
the ciliz may still be seen ona small extent of the front margin close to the 
anterior angles, as may be well observed in Cybister ; these rudiments of ciliz are 
usually confined to an extremely small space, and are very difficult to detect with- 
out dissecting the head from out of the thoracic cavity. The base of the pronotum 
in those genera having a visible scutellum is usually a little sinuate on each side 
near the outer angles ; in those genera where the scutellum is invisible the base is 
a little produced backwards in the middle so that a more or less distinct angle is 
formed. The surface of the pronotum never bears any setee, and has no distinct 
basal or other foveze, but very frequently a more or less distinct transverse series 
of punctures near the front margin but not reaching the sides may be seen, and 
also a second series near the hind margin, but more or less broadly interrupted in 
the middle : on the disc traces of a longitudinal channel may be seen in the shape 
of a fine short impression, and in Lancetes this channel becomes so elongate as to 
extend half the length of the thorax, while in Amphizoa it is entire: in Bidessus 
and in Sternopriscus there exists a peculiar longitudinal plica or fold on each side 
at some considerable distance from the middle. 
On viewing the prothorax from the side it is seen that the sides of the pronotum 
form a more or less conspicuous roof-like expansion; this expansion is a growth of the 
pronotum and does not include any portion of the prosternum, for on looking at the 
under surface it is seen that the junction between the pronotum and the pieces 
of the prosternum is not at the outside edge of the thorax, but entirely on its under 
surface, at a considerable distance from the edge. The prosternum proper isa trans- 
verse piece, connected with the episternum by a suture which is usually very distinct, 
but in Hydroporides is quite obsolete ; its lateral termination connects with the 
side of the pronotum by a very short suture which is frequently quite open in front ; 
although the sides of the prosternum proper are thus very short (in the longitudinal 
direction) the middle portion has a very large development, it takes a downwards. 
and backwards direction between the anterior coxee and is prolonged behind them 
to articulate with the middle of the meso- and metasterna; the prosternum proper 
may therefore be described as consisting of a middle longitudinal piece, and a 
lateral wing on each side of it in front. The longitudinal portion is of much impor- 
