On Aquatic Carnivorous Coleoptera or Dytiscidoe. 225 



The lateral wiiio-s of the medisternum are each very short in the lonoitudinal 

 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 jJart 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 lies 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 Hydro vatus a large part of its area is impressed for the 

 accommodation 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 Notei'ides) 

 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 



