242 On Aquatic Carnivorous Coleoptera or Dytiscide. 
on a raised carina developed near the borders of the ventral plates and the outside 
of the hind coxa, and at the shoulder is received in a groove on the upper edge of 
the metathoracic episternum ; the base rests directly on the vertical mesosternum, 
and the sutural angle is shaped so as to fit under the edge of the raised scutellum ; 
by these means accurate adaptation of the wing-cases to the after body is secured, 
and then the hind margin of the pronotum rests on the base of the elytra, while 
the prothorax itself is firmly kept in its position by its prosternal process being 
locked in the metasternal groove; this latter part in fact plays the part of the 
keystone of the arch. 
Each elytron is articulated to the mesothorax by a projection from the base of 
the former about half way between the suture and outer margin; external to the 
articular point the front margin is turned downwards, till it reaches the humeral 
angle, which is usually more or less prominent and nearly rectangular ; behind the 
shoulder the margin continues deflexed, and persists till towards the hind part, 
gradually becoming narrower, it ceases altogether at some distance before the hind 
sutural angle; this deflexed portion of the wing-case 1s called the epipleura, its 
lower (or inner) edge is bordered by a very fine raised margin, and its junction with 
the upper surface is more or less distinctly marked by a raised edge, which forms 
the external outline of the body (after the prothorax). The epipleura attains its 
maximum of development so far as this family is concerned in some species of 
Megadytes (Cybister costalis, e.g.) while its minimum is found in Eretes and Coelam- 
bus ; and in many of the family the epipleura becomes extremely slender behind 
the middle (Agabus, Ilybius, &c.) In some forms of Hydroporides (Hydrovatini, 
Hyphidrini, Ceelambus, &c.) there is more or less conspicuously marked off at the 
shoulder of the epipleura a triangular space, which I have called the genicular or 
humeral area ; this receives the knees of the front and middle legs and permits 
these parts to be very closely packed away when not in use. The epipleura never 
extends so far as the hinder sutural angle, indeed the extremities of the wing-cases 
always terminate as a thin edge, and thus permit of the protrusion of the hind 
margin of the terminal segment of the hind body ; a supply of air is obtained during 
life by the insect separating these two parts which are protruded at the surface of 
the water, and are closed when under water by accurate apposition ; the inner face 
of the elytra at this apical part is frequently pubescent (Ilybius, Cybister, Dytiscus) , 
the terminal portion of the wing-case is frequently slightly twisted or arched 
(Ilybius) and sometimes also slightly truncate (Lancetes, Dytiscus): the sutural 
angle varies from being about rectangular, (Dytiscus) to very acute (Hydrocanthus). 
On the inner face of the wing-case near the outer margin there frequently exists 
a ridge parallel to the margin, this is frequently absent (Hretes) or but slightly 
developed (Rhantus, Acilius) : this elytral ridge is wanting on the front part of 
the wing-case, only commencing about the middle or behind it, and usually does 
not extend to the extremity, but disappears a considerable distance before that; in 
