208 On Aquatic Carnivorous Coleoptera or Dytiscide. 
sutures are obliterated, so that its extent can only be determined by the two large 
punctures which mark the anterior limits of the lateral gular sutures. 
The eyes in the Dytiscidee are always very large, and are placed at the sides of 
the head, but also encroach largely on its upper and under surfaces; they are very 
smooth and very finely facetted, but in this latter respect there exist some slight 
differences, as may be seen by a comparison of Agabus and Hydrovatus, with Eretes 
and Laccophilus, the facets being distinctly larger in the former genera. When 
looked at from the front of the head the curved outline of the eye is frequently 
much indented or emarginate just behind the antenna, by the side of the front 
part of the epicranium ; but in other cases the outline is preserved nearly intact at 
this point, so that the eye may be described as possessing in front a circular inner 
margin: Agabus and Hydrovatus may be referred to as exhibiting the former, 
Eretes and Cybister as exhibiting the latter structure ; as might be expected inter- 
mediate forms occur as regards this point, such are seen in Laccophilus, and even 
in Dytiscus. 
The labrum is usually quite conspicuous, and placed on the front of the 
epistome or clypeus in such a manner as to continue its curve; but in a few 
genera it is more or less withdrawn to the under surface of the head, and is corres- 
pondingly inconspicuous ; in Queda it is completely concealed, and in Pachydrus, 
and numerous species of Hydrovatus its front margin alone can be seen; in 
Hyphydcus although visible and exserted, it is placed more on the under surface of 
the head ; this position of the labrum depends entirely on the formation of the 
front of the epistome, for the labrum being always attached to the front edge of 
this part, varies in position according as this is inflexed or not ; thus in Queda and 
Pachydrus the front of the epistome is quite doubled or folded under, and thus 
carries the labrum back to the under surface of the head, while in Hyphydrus the 
front of the epistome, though not completely doubled under, is placed at right angles 
to its posterior part, and the labrum assumes a corresponding position ; Coelambus 
is a variable genus as regards the form of the front of the clypeus, and the ex- 
posure of the labrum. In its form the labrum shows but little variation ; it is 
always strongly transverse, greatly broader than long, and its front edge is more 
or less emarginate in the middle, the emargination varying much as regards its 
breadth and depth; the curve of the emargination is fringed with cilize implanted ina 
transverse groove: in Queda where the labrum is concealed, the whole of its anterior 
edge appears to be set with elongate fine cilize, and in Hyphydrus the cilize extend 
nearly across the whole breath of the labrum, but in the great majority of the 
family the rule is that the emargination of the labrum is confined to a comparatively 
small width in the middle, and the cilie are quite short. In the Noterides the 
emargination is very slight, and the ciliz are quite wanting : in Dytiscus latissimus 
the emargination is very slight. The labrum is always yellow in colour, and its 
upper surface is completely destitute of the isolated exserted setee which are so con- 
