204 On Aquatic Carnivorous Coleoptera or Dytiscide. 
Dytiscus lanio) it can also be traced nearly across the head; on the other hand 
in Noterini, Laccophilini, and the Hydroporides, the clypeal suture is totally 
obliterated: in the genus Dytiscus where this suture, as already remarked, 1s 
unusually distinct it is in some species greatly deeper in one sex than in the other 
(vide Dytiscus dauricus, No. 998). The clypeus itself terminates over the labrum 
as a thin edge, so that the labrum continues the plane of the surface of the 
clypeus ; but in Hyphydrus we find an exception to this, the anterior part of the 
clypeus being deflexed at right angles, so that the labrum is placed on a different 
plane to that of the front of the head; in this case a slightly raised line passes in 
a curve from eye to eye, and marks off the small deflexed anterior portion of the 
clypeus from the rest ot the surface; there are other members of the Hydroporides 
in which the deflexed anterior portion of the clypeus exists in a less distinct 
manner, (vide Dytiscus ineequalis and allies in the genus Ceelambus), and this is 
also the case with the raised margin, which exists in various degrees of obliteration ; 
or partial development, the middle part of this line is frequently absent, even 
when the lateral part remains distinct on each side, (vide species of Herophydrus 
and Hydrovatus) ; I have spoken of this margin as existing in various degrees of 
obliteration, but it is doubtful whether this is really the case, and perhaps it may 
rather be that it is in some cases partially developed ; this question can scarcely 
be determined without the aid of embryological research, but as the deflexed and 
margined clypeus is accompanied by a change in position of the parts of the 
mouth, inasmuch as they are in such case placed more on the undersurface, and 
as it seems to be an advantage to these predaceous beetles to have the parts of 
the mouth, more especially the mandibles and maxillee, quite in the anterior part 
of the head, I think the process of evolution is to bring about the diminution of 
the deflexed anterior portion of the head, and so permit the labrum, and the 
mandibles and maxilla: which this covers to be brought quite to the front of the 
head ; but from other considerations, which I need not here enter into, the reverse 
of this might be argued. 
The depressed line, which I have described as existing on each side of the head 
close to the front angle of the clypeus, exhibits much variation, and is in the Hydro- 
porint frequently so indistinct as to be nearly absent; in other cases these lines 
assume a greater extension so as to nearly join in the middle, and form a nearly con- 
tinuous line parallel with the front of the clypeus (Dytiscus maculosus, No. 92, e.g.); 
in those species where the front of the clypeus is deflexed, there may be detected 
at its very front edge a transverse depression, which seems to have arisen from the 
extension of these lines (vide Hyphydrus major); in the Noterides these clypeal 
depressions are usually placed nearer together, and so farther from the angles of 
the clypeus, while in Colpius they appear only to be represented by a very vague 
impression at a rather greater distance from the front edge. In the larger Dytiscide, 
these depressions assume more the form of foveze than of lines, and are of remarkably 
