On Aquatic Carnivorous Coleoptera or Dytiscida. 925 
In short the broad coxal processes are each separated into two distinct parts by 
the coxal notch—an inner adpressed part, and an outer free part. 
This structure is essentially the same as exists in Pelobius ; and in certain Hydro- 
porini the structure is approached in one or more particulars, but in none of the 
Hydroporin: do we find a distinct coxal notch, coexisting with a broad intra-rimal 
and a large extra-rimal part of the coxal process. 
The approximations of other secondary aggregates to the Hydrovatini are impor- 
tant ; the peculiar prosternal process is repeated in some Bidessini and Hyphidrini ; 
and the general form, and the shape of the hind coxe (minus the coxal processes), 
and the swimming legs, is repeated in some Hydroporini, (in certain species ot 
Coelambus) ; the peculiar acuminate hind extremity of the body is reproduced in 
Celina and Methles, two isolated primary aggregates of Dytisci complicati ; while 
Hydrocoptus and Pronoterus, two primitive forms of Noterides in the Dytisci 
fragmentati, not only resemble Hydrovatus to an extraordinary degree in their 
facies or general appearance, but show a very similar structure of the coxal processes, 
These facts render it probable that the Hydrovatini will be found to be creatures 
that have retained to a large extent their primitive characters, while becoming 
highly specialized for a life of very small locomotor activity. If we recall the fact 
that in the Hydrovatini the middle and hind coxee are unusually distant from one 
another, it seems probable that the primitive ancestors of the group, at the period 
when they began to be modified for an aquatic life, were beetles whose coxal cavities 
were separated by a considerably greater interval from one another than were 
those of the ancestors of the other (now existing) Hydroporides. 
The group is widely distributed over the globe, but wanting in the Pacific 
Islands and New Zealand, and does not extend to the cooler parts of the world ; the 
extreme south of England being, so far as is yet known, its farthest extension 
the temperate regions. 
Il. 7.—Group Biprssint. (Vide p. 336.) 
Rather more than one hundred species arranged in six primary aggregates (of 
which however noless than three are autogenera) form this secondary aggregate. 
It is distinguished by a single character not only from other water beetles, but 
from all other beetles, viz., that the hind coxa is soldered completely to the ventral 
segments, so that the under surface of the body from the front of the metasternum 
to the hind margin of the third ventral segment consists of one rigid piece. 
A second important character of the Bidessini is that the posterior articular 
cavities are not contiguous, and are not protected by the coxal processes; they 
open directly on the under surface of the coxee and are separated from one another 
by the coxal processes which are completely adpressed to the level of the ventral 
segments ; in this portion of their structure they are closely approximated by the 
Hyphydrini. 
The hind coxz are always large, with greatly arched anterior border: this — 
6 C2 
