On Aquatic Carnworous Coleoptera or Dytiscide. 237 
are separated by a vertical wall, or plate ; this wall ascends from the common suture 
between the two coxee, and at its termination, spreads out above into a kind of 
winged or bifid process which completely fills up a gap that would otherwise 
exist between the arched roofs of the two cavities. The articular portions of 
the coxa project backwards, and on each side of the projecting portion there is a 
hollow or axilla which permits the base of the swimming leg to be rotated forwards 
till the front edge of the femur can quite attain the longitudinal middle line of the 
body. In the higher and larger forms of the Dytiscidee which I am now describing, 
the articular cavities are entirely concealed by the coxal processes, which project 
from the surface more or less slightly backwards, and are thus very conspicuous on 
the under surface of the body, the swimming legs protruding as it were from the 
hind part of the projection, and being so close together that their inner margins 
touch one another at the point of articulation. This structure, with modifications 
in some details such as the size and form of the coxal lobes, prevails throughout 
the Macro-Dytiscide, without any exception. But in the smaller Dytiscide the 
articulation of the swimming legs is of a different and much less uniform nature. If 
a Hyphydrus be looked at—and with this object one of the two swimming legs 
should be carefully disarticulated—it will be seen that the swimming legs do not 
project from behind and above any prominent coxal processes, and that they 
are not contiguous, but are separated by a considerable space, and if the 
lee has been carefully disarticulated, without damage to the articular cavity 
it will be seen that this is a circular orifice, completely exposed and not concealed 
by any projecting lobe, and a farther examination renders it evident that the space 
separating the articulations of the two sides of the body, consists of the coxal 
processes. Here then we have the articular orifice exposed and placed outside of 
the coxal process, separated from its fellow and circular in form, instead of being 
placed above the coxal process, concealed by it, and contiguous with its fellow ; on 
separating the hind body from the coxa it is farther seen that the upper portion™ of 
the articular box projects further back than the coxal processes, these latter, being 
adpressed to, and soldered to the former: thus the coxal processes, are not prominent 
from the level of the base of the abdomen, and there is no axilla formed to permit 
the flexion of the leg forwards : indeed the articular cavity being circular and exposed 
permits the leg to be rotated forwards even farther and more freely than in the 
Macro-Dytiscidee. This is the type of structure which admits of the greatest amount 
of motion of rotation for the swimming leg and it attains its maximum of develop- 
ment in the New-World genus Pachydrus, where it is accompanied by a consolidation 
of the coxa with the abdominal segments. Many of the Hydroporini have the 
articular cavities constructed in a manner that is intermediate between that just 
described, and that of the Macro-Dytiscide ; as a good instance of an intermediate 
* IT have in my descriptions occasionally spoken of this upper portion of the articular cavity, when it is 
visible under the name “ pyxal process.” 
21? 
