Oil. A(iuatic Caraioorous Coleoptera or DytAscidte. 967 



characters In classifying the Coleoptera, and although the Hydroporides in the 

 D3'tiscid8e, agree in this respect with the tetraraerous Coleoptei^a, yet it is perfectly 

 clear that this similaritj' of structure has been arrived at by perfectly distinct 

 lines of descent ; and if we are not justified in considering this point of similarity 

 in the Dytiscidas and Tetramera as evidence of community of descent between 

 them, on what logical grounds is it thit we may consider it as being such in the 

 case of the various members of the Phytouhaga ? 



The Dytisci complicati are found in all parts of the world, but in New Zealand and 

 the Polynesian islands they are, so far as is yet kno wn, very poorly represented, although 

 Australia and Tasmania are very rich in species and genera. They appear to be 

 remarkably tolerant of cold, the proportion of species found in high latitudes in 

 the Northern hemisphere being remarkable : the series in this respect offers there- 

 fore a great contrast to the Dytisci fragmentati, where the vast majority of the 

 species are found in the warmer regions of the earth, and very few indeed, if any, 

 penetrate to near the Arctic regions or are alpine in their habits. 



FIFTH SYNTHESIS, 



The Family DYTISCID^E. 



The family as comprised in this work consists of the two unequal series, 

 Dytisci fragmentati, and Dytisci complicati, the former consisting of only 175, 

 the latter of about one thousand species. 



A detailed account of the external structure of the familjr has been given at the 

 commencement of the work (pp. 190 et seq.), and the definition of the family 

 generalized from those details at p. 257. To complete the account it is necessary 

 for me to make some remarks on the points of approximation or similarity between 

 the Dytiscidse and other families of the order Coleoptera. 



The points of community between the Carabidse and DytiscidaB are so numerous 

 and important, that it is not possible to point to any one character as an absolute 

 test of distinction between the two families ; and although several peculiarities 

 may be pointed out as being especially characteristic of the Dytiscidte, yet all of 

 them when taken seriatim disappear within the bounds of the family or are found 

 in one or more members of the Carabidee. These characters may be enumerated as 

 follows : 1, the peculiar, continuous outline and solid compact form ; 2, the short 

 broad head, immersed in the thorax as far as the back of the eyes and not 

 narrowed behind the eyes ; 3, the glabrous antennae, free from setae and sensitive 

 pubescence ; 4, the constant existence of a prosternal process ; 5, the fact that 

 this prosternal process articulates, or rather connects with the metasternum ; 6, 

 the extremely lai'ge external lamina of the hind coxa ; 7, the accurate adaptation 



