On Aquatic Carnivorous Coleoptera or Dytiscide. 219 
the special characters of the family. So that the epoch to which we must remount 
before we can imagine a common ancestor for these two allied genera is enormously 
remote: while to believe that there may have been a common ancestor for the 
whole of the Dytiscidze becomes almost impossible. 
The anterior coxal cavities are placed as a kind of excavation, one on each side of 
the middle piece of the prosternum, and vary in shape in conformity with the coxee ; 
they are imperfect inasmuch as they only very partially enclose the coxee, especially 
in those forms where the coxe are elongate as in Hyphydrus; owing to their 
peculiar form and position, the cavities appear to be open behind, but this is not 
really the case, and in fact all the Dytiscidse have the coxal cavities closed behind ; 
the closing of the cavities is effected, as usual, by an expansion given off from either 
side of the posterior aspect or edge of the middle piece of the prosternum ; this 
expansion is in the Dytiscidze nearly colourless, and might be supposed to be 
membranous, or semimembranous in consistency, but it is really quite corncous : 
moreover it does not connect together the posterior extremities of the side pieces as 
is usual in the Coleoptera with closed coxal cavities, but is placed as it were somewhat 
in the interior of the prothorax, and so leaves the angles of the side pieces free and 
prominent ; this mechanism is highly important as it is, im conjunction with the 
articulation between the prosternal process and the afterbody, the means of 
securing a perfect articulation between the prothorax and the afterbody, and 
consequently of excluding water from the interior of the insect ; the prominent 
transverse portion of the bridge closing the coxal cavities, actually enters into 
the interior of the mesosternum (of which it has exactly the width) while on each 
side there is left a groove or depression into which the front edge of the 
mesosternum fits, the free edge of the prothoracie side piece playing over that of 
the mesothoracie piece. The coadaptation of the various parts of the posterior 
aspect of the prothorax, to corresponding parts of the afterbody and base of the 
wing-cases is extremely perfect and very complicated; proceeding from below 
upwards we have, first, the prosternal process stretching beyond the mesosternum 
to be received in a metasternal groove ; directly above the prosternal process we 
see a considerable protuberance or prominence which fits into the fork of the 
mesosternum ; then come the posterior aspects of the coxee which fit into facetts 
on the face of the mesosternum, and on a still higher level we have the transverse 
bridge closing the coxal cavities which fits into the interior of the mesosternum, 
while on the upper surface we find that the base of the mesothorax and scutellum 
are shaped so as to allow the hind margin of the pronotum to overlap and 
accurately fit them, while the shoulders of the wing cases are prominent, and rest 
on an expansion of the posterior face of the pronotum which is beautifully sinuate 
and emarginate to facilitate the coadaptation. This jointing is so perfect in the 
higher forms such as Cybister, that if after the prothorax has been detached from 
the afterbody an attempt be made to replace it in its natural position, this 
TRANS. ROY. DUB. SOC., N.S., VOL. II. 2G 
