232 On Aquatic Carnivorous Coleoptera or Dytiscide. 
extremely the appearance of being a distinct piece of the sternum, which however 
is not the case. The upper or outer portion of the episternum is, at any rate in 
its anterior portion, covered by the wing-case, and when the part covered by the 
wing-case is large, as in Hydrovatus, then the posterior extremity appears very 
slender and acuminate, but on the elytra being taken off it will be seen that in: 
such cases the termination of the episternum is really obtuse, the greater portion 
of its area being turned upwards at an angle with the exposed portion; this 
covered portion of the episternum may readily be mistaken for the epimeron, 
more especially as it is of a different texture from the exposed part, and is 
traversed by a sort of line, having somewhat the appearance of a suture, between 
the covered and exposed portions. The epimeron of the metathorax, unlike the 
episternum—is very indistinct, and might on a careless inspection, be supposed to 
be absent; it is however always present and is placed on the upper posterior 
portion of the episternum, but extends farther backwards, than this piece does ;. 
the suture between the episternum and epimeron is covered by the wing-case, and — 
is often very fine and indistinct, but is always conspicuous when the part is 
properly cleaned and exposed ; the episternum towards its extremity becomes, as 
above stated, narrower, and thus trenches away from the edge of the wing-case, 
and an angle is thus formed between it and the upper and outer portion of the 
hind coxee, and into this angle the epimeron protrudes, so that at this point it 
approaches more to the under surface than it does elsewhere, and in many forms 
(especially in Eretes, Thermonectini, and Cybistrini,) the epimeron becomes visible, 
even when the wing-cases are closed, as an acutely angular projection at the upper 
and hinder angle of the apex of the episternum. Behind this point the epimeron 
again completely covered by the wing-case, proceeds farther backwards along the 
external or upper part of the hind coxa, till it terminates behind by connecting 
with the side of the basal segment of the abdomen or hind body; sometimes 
the connexion between these two parts is effected not by actual contact, but by 
means of an intervening membranous space (Acilius, Hydaticus, and many others, 
but in other cases (Hyphydrus, Eretes) there is complete contact between the 
horny portions of the two parts; between the extreme degrees of separation (as 
seen for instance in Coptotomus) and the absolute contact of Hyphydrus and 
Eretes, there exist every grade of connexion; thus in Agabus it would be almost 
equally correct to describe the two parts as separated by a membrane or as in 
contact. The upper portion of the epimeron becomes membranous, and connects 
with the pieces of the metanotum. In those cases where the middle coxe are 
widely separated, and there is no mobility of the pieces of the thorax (Hydrovatus,. 
Hydrocanthus, Pachydrus) there is a most extreme and perfect adaptation of the 
inflexed edge of the epipleura, to the side of the body, and in these cases the 
epimeron remains nearly entirely membranous, a mere strip of semicorneous matter, 
along its lower edge and its extremity, serving for the suture with the episternum 
