932 On Aquatic Carnivorous Coleoptera or Dytiscide. 
and in groups 7 and 8 of the genus Agabus, they are scarcely present, there being 
merely a few punctures grouped close together in a manner approximating to 
rectilinear, and each bearing a very obscure short hair or cilia. Where the swim- 
ming leys are highly developed (comp. Agabus, group 4), the punctures become 
confluent so as to form a regular line or depression, in which are placed the thick, 
contiguous, and regularly arranged ciliz. In all cases however, including even 
those species where their development is rudimentary, the cilize are placed so as to 
form at their insertion a kind of linear depression parallel with and approximate to 
the hind border of the femur at its outer extremity. 
The width of the ventral side pieces is subject to some degree of variation, but 
so far as I have been able to examine the character, the variation is not great ; and 
it may be said that the width of the ventral side piece of the fourth segment 
is about one-half or one-third of the length. 
The aggregate may be described as a really natural one, inasmuch as the definition 
given of it will apply to no member of any allied group; and also because that 
the primary aggregates of which it is composed are closely linked together. 
There is still however doubt as to its being actually isolated, because if other 
characters besides those above enumerated be taken into consideration, then a fresh 
combination becomes possible. Moreover each of the characters reappears, or at 
any rate is greatly approximated to, in allied groups. Thus the wide ventral side 
pieces reappear in Scutopterus, (hw. op.) although the rest of the members of the 
group with which that genus is associated (the Colymbetini) have the ventral side 
pieces narrow. As an instance of an important character that is variable in the 
group, but which has been left out of consideration by me, I may point out the 
penultimate stigma; this in Agabus is quite small, while in Colymbetini it is 
transversely elongate. In Ilybius which I have included among Agabini, this 
stigma is also transversely elongate to as great an extent as it is in certain 
Colymbetini. 
C, J. Thomson, (and other naturalists have tollowed him in this), has made the 
want of a setigerous space on each of the middle of the third, fourth, and fifth ventral 
plates a distinctive character of the European Agabini, and has thus distinguished 
the group from the Colymbetini, in which the setigerous pore is present in a high 
degree of development. The character will not however bear when rigidly examined 
so high a degree of taxonomic importance as that assigned toit by the talented and 
most observant Swedish naturalist; for on careful examination it may be found 
present in Agabini in various stages of development. Thus in Dytiscus bipustu- 
latus (Agabus No. 751), a few scattered fine punctures bearing a very short hair may 
be detected about the middle of these segments, and in Dytiscus fuscipennis (Agabus 
No. 752) these punctures and sete are more highly developed, and are placed so 
as to form a diffuse patch on each side of the middle of the segment: in Ilybius they 
are present to a greater or less degree of concentration on the middle of the segment; 
