216 On Aquatic Carnivorous Coleoptera or Dytiscidw. 



The side of the pronotum is most frequently Hmited above by a raised margin ; 

 this margin may be broad as in Hyderodes, or very fine as m Eretes, or may 

 be entirely absent, as in the Cybistrini and numerous others, and all the grades of 

 connection exist between these extreme instances. In a 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 Dytiscidte it is destitute of either membrane or cilire, except that frequently 

 the ciliae may still be seen on a small extent of the front margin close to the 

 anterior angles, as may be well observed in Cybister ; these rudiments of cilite 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 setae, and has no distinct 

 basal or other fovete, but very frequentl}^ 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 ihe length of the thorax, while in Amphizoa it is entire : in Bidessus 

 and in Sternoprlscus there exists a peculiar longitudinal plica or fold on each side 

 at some considerable distance from the middle. 



On \iewing 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 is a 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 coxae 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 jiortion is of much impor- 



