224 On Aquatic Carnivorous Coleojjtcra or DijtiscidcB. 



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 foi'k 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 pi'ominent (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 (D^^tiscus and many other 

 Macro-Dytiscidas), while in other cases the metasternal process is sufficiently 

 perfected to articulate completely wath the mesosternal fork without this latter 

 being furnished with any backward prolongation (Cybister). Although the 

 mesosternal fork is in the great majority of the Dytiscidse connected with the 

 metasternum ; yet in Pelobius, in the Vatellini, iu a large portion of Hydroporini, 

 and in Sternopriscus this is not the case, and the middle piece of the medisternuiii 

 does not connect at all with the metasternum : this is of considerable importance, 

 for in the Carabidae 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 Dytlscidpe, is greater than in the Carabidre, while on the other 

 hand the corresponding part of the mesosternum is smaller than in the Carabidae, 

 though very highly modified in other resj^ects. 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 tow'ards 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, Hyphydrus). 

 It may also be remarked that the perfection of this connexion is to a considerable 

 extent correlative with the distance between the intermediate coxse, as when these 

 are very close together, and more especially when they are also prominent fi'om 

 their cavities (Vatellini, Sternopriscus,) then the connexion is wanting, while when 

 the coxse are well separated (Hydrovatus, Noterides, most Hydaticides) then the 

 conne>:ion is perfect. The side of the middle column is distinct in most of the 

 Macro-Dytiscidse (see Dytiscus, Meladema, Ilybius), 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 coxae ; 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 coxae. 



