On Aquatic Carnivorous Coleoptera or Dytiscide. 225 
The lateral wings of the medisternum are each very short in the longitudinal 
direction, and extend on each side so as to assist in forming the anterior portion 
of the middle coxal cavity ; they become each a little longer towards the outside 
of the coxa, where they terminate by the junction of the outer extremity with the 
lower portion of the epimeron ; throughout the family they are very constant 
showing no important variations in size and form. 
The larger portion of the area of the mesosternum is formed by the episterna ; 
each of which is a large piece rising from the upper edge of the side-wing of the 
medisternum directly over the middle coxa ; its upper extremity is shorter (in the 
true longitudinal axis) than the lower, and rises higher than the other parts of the 
mesosternum and plays an important part in the articulation of the wing-case ; the 
inner margin of the episternum is the border of the large foramen affording the 
means of communication between the prothorax and afterbody; this inner margin has 
the appearance of being a distinct piece, articulating at its lower part with the front 
piece of the middle column of the medisternum (referred to above as being probably 
a distinct piece) ; at its upper extremity this inner margin expands and becomes 
hollowed, in a more or less evident manner, and thus affords space for the expansion 
or play of the prothoracic stigma which les immediately in front of it. The 
episternum although forming so large a portion of the mesosternum is of little 
taxonomic importance, as it varies very little throughout the family, it is remarkably 
large in the Noterides, and in Hydrovatusa large part of its area is impressed for the 
avcommodation of the middle femur. It is on the episternum that the torsion by 
which the mesosternum becomes diaphragm-like, is effected ; the front edge always 
preserves the natural direction, but the episternum is more or less gradually bent, 
so that the hind part of the mesosternum is placed something like at right angles 
to the front edge, which thus appears to form a very short neck. 
The epimeron of the mesosternum is always smaller than the episternum, and its 
lower extremity always penetrates to the middle coxal cavity ; its inner (more 
correctly anterior) margin is closely connected with the episternum, the suture 
between the two being fine, but usually distinct, but in Bidessus and Sternopriscus 
it is most frequently very indistinct: the outer margin is applied to the front edge 
of the metathoracic episternum. The epimeron is always very short (in the truly 
longitudinal direction) at its lower part, but usually becomes broader as it reaches 
to the shoulder, and after its angle of junction with the humeral angle of the 
metathoracic episternum it again becomes narrower, so that it forms a rather 
irregular, narrow and elongate triangle; in some cases, however, the epimeron 
becomes scarcely any broader at the metathoracic angle (Bidessus and Noterides) 
so that it may be described as linear, although in point of fact it always shows a 
well marked, sharply defined angle at its point of junction with the angle of the 
metathoracic episternum. In the Noterides the linear form of the epimeron is 
accompanied by a very large development of the episternum, but this is not the case 
