220 On Aquatic Carnivorous Coleoptera or Dytiscide. 
is very easily effected; and it will then be found that the thorax retains its 
position in spite of considerable efforts being made to dislodge it; 1t may however 
be instantly separated by the point of a needle being thrust under the apex of 
the prosternal process. The most completely fixed prothorax is found in 
the Noterides, and more specially in the genus Hydrocanthus, where the 
excessively broad, truncate prosternal process is so accurately fitted to the 
metasternum, that the two parts look as if they were soldered together. The 
epimera and episterna of the prothorax sometimes exist in the Dytiscide 
with their sutures distinct (as in Acilius and Cybister) so that the shapes and the 
comparative areas of the two pieces can be readily distinguished ; but in the Hydropo- 
rides the sutures are usually obsolete. When they can be distinguished it is seen that 
the larger part of the side of the prosternum is formed by the episternum, the 
epimeron being a comparatively linear piece : the front coxal cavity is formed how- 
ever by the junction of the epimeron with the anterior band of the prosternum 
proper; the epimeron sends off a prolongation for this purpose along the border of 
the coxal cavity, which entirely cuts off the episternum from forming any part of 
the articular cavity. 
The structure of the prothorax in the Dytiscide is essentially similar to that of 
the Carabidee, the most important differences being the great development and per- 
fection of the prosternal process, and the mode in which the coxal cavities are closed 
behind. There are some Carabidee which possess, however, a highly developed pro- 
sternal process like the Dytiscidee (vide especially Cyclosomus): and in Trachypachys 
and Systolosoma of the Carabide, the structure of the prothorax approaches very 
much to what exists in the Dytiscidee, and indeed shows comparatively little differ- 
ence from that of Amphizoa. These three genera, Trachypachys, Systolosoma and 
Amphizoa may truly be said to show a structure of the prothorax intermediate 
between that of the Dytiscidee and Carabide. The prothorax of Pelobius is very 
different—quite different one might say—from that of Amphizoa, Trachypachys and 
Svstolosoma, and in certain respects is formed like that of the Carabidze, though on 
the whole it must be pronounced to be a very different prothorax from that of any of 
the Dytiscide or Carabide ; its general form, apart from its acuminate and large and 
remarkably elongate prosternal process, is rather that of the Carabidee than of the 
Dytiscide, andthe very conspicuously ciliate front margins are found in no other Dytis- 
cide but are the rule in Carabide ; leaving the general shape and prosternal process 
out of consideration we find that it has the characters of the Dytiscid prothorax, but 
in a quite peculiar form; the middle band of the prosternum assumes immediately at 
its front margin a directly vertical direction, and then curves backwards arching over 
the coxee which are very elongate ; this marked prominence and incrassation of the 
middle of the prosternum is found only in some of the higher Dytiscidz, such as the 
Cybistrini ; the intercoxal band shows on each side a raised margin, which is quite 
independent of the usual margin of the coxal cavity and prosternal process, but ap- 
pears to be really similar to the fine raised margin found ina similar position in Cy- 
