On Aquatic Carnivorous Coleoptera or Dytiscide. 247 
this triangle is usually broadly rounded off; and as the mner edge of the tibia is 
longer than the outer one, the apex is oblique in its direction: the articular cavity 
for the tarsus cannot be seen from the front, being placed always on the posterior 
face of the tibia, and as the result of this the extremity of the tibia in front is a 
more or less chisel-like, or cutting, edge ; this edge beg however always fringed 
with cilize, setze, or spines. The tibize are very greatly modified in form according 
to the sex ; in the Dytiscini, Cybistrini and Hydaticides, the tibie are dilated, and 
in addition a greater or less extent of the outer part of their lower face is cut away, 
so as to create a hollow, which permits of the rotation of the dilated tarsus when 
its convex upper face is turned back, or when the tibia is pressed down on the 
tarsus: and the sexual change in form is accompanied by modifications in other 
respects as will be presently mentioned. The front face of the tibia is punctate 
and its posterior one smooth; the outer edge bears setze, spines, or cili@ of various 
lengths, and degrees of coarseness, and the front of the inner face usually bears 
a dense series of extremely short ciliz. The extremity of the tibia is, as already 
remarked, not a truncature, but an edge, owing to the lower portion of the back 
of the tibia bemg more or less cut away, so that here there is seen a smooth 
polished space, and in this space is placed the cavity for the articulation of the 
tibia ; it is usually very near the extremity and quite close to the inner angle, but 
when the tibia becomes much modified in form (as in the males of Cybister) the 
cavity may be situated nearer to the outer than to the inner margin. Usually it 
isa simple circular depression but in Cybister ¢ the lips of the orifice become 
prolonged into a circular neck, and a similar modification, though to a less extent, 
occurs in other genera where the males have highly modified tarsi. The hollowed 
space on the back of the tibia is an adaptation to the form of the tarsus and may 
be called the tarsal area. This tarsal area varies in size, as the tarsus varies in 
its development, as may be well seen by comparing a species of Cybister where 
the male tarsus is of small size (No. 1122) with another where it is large (C. owas 
No. 1135). The tibial spurs also vary greatly, they may be two in number, 
~ but so small and inconspicuous as to be readily overlooked, this is the case in the 
Hydrovatini; and indeed in the whole family Hydroporides the spurs of the 
front tibize remain small and in this case are inserted at the inner margin of the 
tibia one at its front the other at its hind edge; that in the latter situation 
being longer than the other and placed a little higher up on the tibia. In the 
larger Dytiscidze the spurs are much more developed and conspicuous, and in the 
females are placed in a similar position to that they occupy in the Hydroporides, 
but in the males where the tibia has much changed its form the spurs are 
apparently much changed in their position; thus in Cybister ¢ the anterior 
spur is placed quite at the extremity of the tibia while the posterior one is far 
up on the inner edge; in Dytiscus ¢ the anterior spur is altogether absent, and 
the posterior one is placed on the inner edge a good distance above the extremity ; 
