224 On Aquatic Carnivorous Coleoptera or Dytiscide. 
however the facettes are nearly or quite absent so that the front of the column is 
comparatively plane ; at its lowest point the column is very deeply channelled or 
impressed, to receive a protuberance of the posterior part of the prosternum, the 
sides of the channel being frequently divergent so that a kind of fork is formed 
(mesosternal fork); the front extremity of the fork is usually a slight angular 
prominence, and the hinder termination of the fork or channel, is sometimes also 
angularly prominent (Dytiscus, Cybister, &c.); the floor (or roof) of the channel 
behind usually connects with the intercoxal process of the metasternum, and 
sometimes is prolonged backwards to effect this junction (Dytiscus and many other 
Macro-Dytiscidee), while in other cases the metasternal process is sufficiently 
perfected to articulate completely with the mesosternal fork withont this latter 
being furnished with any backward prolongation (Cybister). Although the 
mesosternal fork is in the great majority of the Dytiscide connected with the 
metasternum ; yet in Pelobius, in the Vatellini, in a large portion of Hydroporini, 
and in Sternopriscus this is not the case, and the middle piece of the medisternum 
does not connect at all with the metasternum: this is of considerable importance, 
for in the Carabidee these pieces seem, so far as I know, to be always connected 
and are sometimes even soldered together (Anthia) ; the development of the pros- 
ternal process in the Dytiscide, is greater than in the Carabidee, while on the other 
hand the corresponding part of the mesosternum is smaller than in the Carabide, 
though very highly modified in other respects. It is well worthy of remark that 
even where the mesosternum fails to connect with the metasternumthere is frequently 
(Deronectes) a growth backwards from it towards the nearest part of the meta- 
sternal process, as if a connexion were in process of evolution (for remarks on this, 
vide last part), Sometimes the apex of the medisternal column does not reach so 
far down as the front of the metasternum, and the connexion between it and the 
metasternum, if such exists at all, is comparatively imperfect (Eretes, Hyphyarus). 
It may also be remarked that the perfection of this connexion is to a considerable 
extent correlative with the distance between the intermediate coxze, as when these 
are very close together, and more especially when they are also prominent from 
their cavities (Vatellini, Sternopriscus,) then the connexion is wanting, while when 
the coxee are well separated (Hydrovatus, Noterides, most Hydaticides) then the 
connexion is perfect. The side of the middle column is distinct in most of the 
Macro-Dytiscidee (see Dytiscus, Meladema, Hybius), but when the mesothorax has 
lost almost absolutely its extension in the longitudinal direction and become merely 
a diaphragm between the meta- and prothoraces (Laccophilus, Hydrovatus, 
Hyphydrus), then the column is so compressed that it becomes a mere lamina, 
bearing two facettes for the accommodation of the front coxee ; the two sides which 
form the back of the column are quite concealed so that they can only be viewed 
by disarticulating the mesothorax, they form a part of the anterior portion of the 
articular cavities for the middle coxe. 
