On Aquatic Carnivorous Coleoptem or Dytiscidce. 965 



now in Colymbetes, at a distance of about 1 or 2 m.m. from the cavity, this edge is 

 completely overwrapped and concealed by the mesothoracic epimeron ; a little 

 nearer to the cavity the epimeron however again diverges and exposes a part of 

 the episternal edge, but close to the cavity the epimeron gives off a process which 

 appears to unite with the side wing of the metasternum by a minute point, and thus 

 to exclude the episternum from the cavity ; this is not however really the case for 

 if the sutures be forced open so that the minute relations of the parts can be clearly 

 seen, it will be perceived that, by the growth of the process of the epimeron just 

 alluded to, the edge of the episternum has been twisted to one side and forced into 

 the cavity in such a way as to leave only a thin lamina of the episternal edge 

 visibly interposed between it and the metasternum ; this apparent exception there- 

 fore shows actually how remarkably pertinacious is the point of structure I have 

 alluded to. The varied aggregates composing the Dytisci complicati are connected 

 together in an intricate manner by numerous other points of structure, no one of which 

 however is constantly present ; except that the posterior coxse possess a characteristic 

 but variable form, their greatest anterior extension being near to the epipleura and 

 far from the mesial line of the body (longitudinally). The remarkable Amphizoa 

 forms however quite an exception to this integration of the Dytisci complicati ; it 

 possesses certain peculiarities found in no other member of the aggregate, and it 

 lacks the peculiar form of the hind coxae, and some other less persistent points of 

 the other Dytisci complicati ; it is in fact quite isolated, and should perhaps not be 

 united with the aggregate in the same synthesis as that in which the other component 

 members are associated, but a special more distant (viz. 5th.) synthesis should be 

 used for this purpose. 



Putting Amphizoa on one side, the aggregate is a very natural one, and although 

 at first sight its tetramerous section appears very different from the pentamerous one, 

 yet there exist some intermediate forms of the highest interest ; such an one is 

 Celina of the Hydroporides, and still more the genus Methles, which cannot be 

 associated in any of the larger syntheses, but remains an isolated form and a 

 remarkable synthetic type. 



Although this is the case, yet careful examination fails to confirm any suspicion 

 of genetic relationship between any of the components of the aggregate. For 

 instance the Hydaticides are distinguished from the Colymbetides by the presence 

 of a peculiar ciliation of the hind margins of the swimming feet ; this is a speciali- 

 zation of high interest, and it might for several different reasons be suggested that 

 the Colymbetides stood in the relation of an ancestral group to the Hydaticides, 

 that in fact Hydaticides were merely Colymbetides that had gained the said ciliation. 

 There exists however a Hydaticid in which this ciliation appears in a rudimentary 

 form. I allude to the genus Eretes. Here then we should expect the connecting 

 link, at any rate, or evidences of a former connection between the Hydaticides and 

 Colymbetides to be found ; yet so far from this being the case Eretes is a most 



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