230 On Aquatic Carnivorous Coleoptera or Dytiscide. 
Hydrocanthini and the Suphisini, the intercoxal process is very large, being both 
elongate and broad, and is nearly truncate in front, in adapatation to the broad 
truncate extremity of the prosternal process, the two edges of these pieces fitting 
together at their line of junction ina most perfect manner : usually the intercoxal 
process is impressed to receive the extremity of the prosternal process, and the 
metasternal groove thus formed, coincides in shape with that of the point of the 
prosternal process ; when the intercoxal process is very broad, (Hydrocanthini, 
Suphisini, Hydrovatini, Pachydrus) then it bears no groove, for the prosternal 
process is brought very closely into adaptation with the intercoxal process; and 
there is in these cases excessively little mobility of the prothorax, these being the 
forms in which this characteristic of the Dytiscide has become almost absolute ; 
when the intercoxal process is very narrow, (as in many Agabi, especially group 
12; Eretes, &c.,) then the groove is elongate, narrow, and indefinite, consisting of 
a mere depression along the middle of the process; and when the prosternal process 
is short, feeble, and acuminate, which usually occurs in such forms as have the middle 
coxee rather approximate, and the intercoxal process more or less curved upwards 
at its extremity (as in Hyphydrus, Ceelambus, and other Hydroporini in which the 
mesosternal fork is not connected with the intercoxal process) then it can scarcely 
be said that there is any metasternal groove present; while its maximum of per- 
fection is attained in the Cybistrini, where it is deep, with very definite sides, and 
perfectly adapted to the prosternal process, but yet when the point of the process is 
slightly lifted allows some movement of extension between the prothorax and after- 
body. 
The central piece of the metasternum, is more or less produced behind between 
the front parts of the hind coxee, its sides usually meeting to form a more or less 
acute angle ; in Hyphydrus however the posterior portion of the metasternum does 
not form an angle, but has a comparatively broad rounded extremity. From the 
front of the intercoxal process to its posterior angle the metasternum is usually 
elongate, but varies much in this respect and in Hydrovatus it is short and conse- 
quently its posterior angle is obtuse ; the greatest reduction of the middle of the 
metasternum occurs in the short globose Dytiscide, and is exhibited by Suphis and 
Colpius, where its posterior part has scarcely any backward prolongation in the 
middle. Nearly always there is a fine longitudinal channel running along the 
middle of the metasternum, but this is frequently very obsolete. The most peculiar 
character exhibited by the middle of the metasternum, is that in the Noterimi and 
Hydrocanthini, it is more or less raised or prominent, so as to form in conjunction 
with the prosternal process, and the middle portions of the hind coxee, a kind of 
broad flat keel, increasing in width as it progresses backwards; this peculiar structure 
reaches its greatest development in Hydrocanthus, and is without any parallel 
in other beetles. The laciniz or side wings of the metasternum are as it were 
compressed between the posterior coxze, lying behind them, and the episternum lying 
