220 On Aquatic Carnivorous Coleoptera or DijtisciclfB. 



s very easily effected ; and it will then be found that the thorax retains its 

 position in spite of considerable efforts being made to dislodge it ; it may however 

 be instantly separated by the point of a needle being thrust under the apex of 

 the prosternal process. The most completely fixed prothorax is found in 

 the Noterides, and more specially in the genus Hydrocanthus, where the 

 excessively broad, truncate prosternal process is so accurately fitted to the 

 metasternum, that the two parts look as if they were soldered together. The 

 epimera and episterna of the prothorax sometimes exist in the Dytiscidae 

 with their sutures distinct (as in Acilius and Cybister) so that the shapes and the 

 comparative areas of the two pieces can be readily distinguished ; but in the Hydropo- 

 rides the sutures are usually obsolete. When they can be distinguished it is seen that 

 the larger part of the side of the prosternum is formed by the episternum, the 

 epimeron being a comparatively linear piece : the front coxal cavity is formed how- 

 ever by the junction of the epimeron wnth the anterior band of the prosternum 

 proper; the epimeron sends off a prolongation for this purpose along the border of 

 the coxal cavity, which entirely cuts off the episternum from forming any part of 

 the articular cavity. 



The structure of the prothorax in the Dytiscidse is essentially similar to that of 

 the Carabidse, the most important differences being the great development and per- 

 fection of the prosternal process, and the mode in which the coxal cavities are closed 

 behind. There are some Carabidse which possess, however, a highly developed pro- 

 sternal process like the Dytiscidae {vide especially Cyclosomus): and in Trachypachys 

 and Systolosoma of the Carabidfe, the structure of the prothorax approaches very 

 much to what exists in the Dytiscidse, and indeed shows comparatively little differ- 

 ence from that of Amphizoa. These three genera, Trachypachys, Systolosoma and 

 Amphizoa may truly be said to show a structure of the prothorax intermediate 

 between that of the Dytiscidfe and Carabida?. The prothorax of Pelobius is very 

 different — quite different one might say — from that of Amphizoa, Trachypachys and 

 Systolosoma, and in certain respects is formed like that of the Carabida?, though on 

 the whole it must be pronounced to be a very different prothorax from that of any of 

 the Dytiscidse or Carabidse; its general form, apart from its acuminate and large and 

 remarkably elongate prosternal process, is rather that of the Carabidse than of the 

 Dytiscidse, and the very conspicuously ciliate front margins are found in no other Dytis- 

 cidse but are the rule in Carabidce ; leaving the general shape and prosternal process 

 out of consideration we find that it has the characters of the Dytiscid prothorax, but 

 in a quite peculiar form; the middle band of the prosternum assumes immediately at 

 its front margin a directly vertical direction, and then curves backwards arching over 

 the coxse which are very elongate ; this marked prominence and incrassation of the 

 middle of the prosternum is found only in some of the higher Dytiscidse, such as the 

 Cybistrini ; the intercoxal band shows on each side a raised margin, which is quite 

 independent of the usual margin of the coxal cavity and prosternal process, but ap- 

 pears to be really similar to the fine raised margin found in a similar position in Cy- 



