On Aquatic Carnivorous Coleoptera or Dytiscidce. 949 



of the Noterides — appears to be true of all the species of the tribe, but it is 

 likewise found insomeBidessini, in Sternopriscus, and even (though not so remarkably) 

 in Pelobius; no\v' as all these widely different forms have in common the fact that 

 the Tnesosternuiii is placed on a plane more continuous with the plane of the meta- 

 sternum than it is in other Dytiscidce, it is clear that the small size of the meso- 

 thoracic epimeron is correlative with the less amount of change of position of the 

 mesosternum, or in other words, just so much as the mesosternum has been bent 

 towards the metasternum, just so much has the epimeron increased in size : 

 although Bidessus and Sternopriscus resemble Noterides in the small size of the 

 epimeron, they do not agree with it in its comparative obliteration at its upper or 

 episternal angle ; this is clearl}^ because growth of the mesothoracic episternum 

 has gone on in Noterides (owing to greater use of the middle legs in locomotion), 

 while no such growth has taken place in Bidessus and Sternopriscus. 'i'hus we see 

 that the form of the mesothoracic epimeron in the Noterides is the result of the little 

 change of plane of the mesosternum plus the unusual growth of the episternum ; 

 and we can clearly see that these peculiarities are properly considered as amounting 

 to evolution from more primitive to higher forms, and we see further that the 

 likeness presented by other water beetles in this respect is not due to any genetic 

 connection or common ancestry, but rather should be looked on as community 

 of structure owing to similarities of evolution. 



Another peculiarity of the Noterides, viz., the relation between the prosternal 

 process and metasternum, need not occupy us long ; it is very striking in Hydro- 

 canthini, but much less so in the other groups, and all that need be said is that 

 whereas in Hydrojiorides and some other groups accurate adaptation of these 

 parts has been gained com^oaratively late in the process of evolution, on the other 

 hand, in the Noterides it has been a striking feature even in some of the early 

 forms. Sternopriscus, which has been just alluded to as approximating to Noterides 

 by one part of the structure, is in this resjiect abruptly different, it being one of 

 the forms where co-adaptation between the prosternal process and metasternum is 

 most incomplete. 



The unusual development of the spur at the apex of the front tibia, is not present 

 in all the forms, it seems to be absolutely deficient (so far as I can see) in Hydro- 

 coptus and Notomicrus, but it is present in all the higher forms, and in some of the 

 lower ones, and attains its greatest evolution in Suphisini, which in some other 

 respects are to be considered low or primitive forms. 



Although, so far as we know of them at present, and probably really at the present 

 epoch, the Noterides form a very fragmentary tribe, yet it is one which is very well 

 adapted for studying in connection with eA'olution ; and I have come to the 

 conclusion that certainly there is no ancestral relation between the different 

 forms comprised in it ; after careful consideration I feel sure that no species, 

 of it can be considered the ancestor of any other species, no genus the ancestor (jf 



6 F 2 



