226 On Aquatic Carnivorous Coleoptera or Dytiscide. 
in Bidessus, where the episternum and epimeron are each small in area; the 
Noterides are therefore distinguished by the sublinear epimeron and the fact that 
this is very small in comparison with the episternum. Although the epimeron 1s 
always smaller than the episternum, yet in the higher forms it increases so much 
in size that it becomes almost equal to the episternum in area (vide Laccophilus, 
Acilius, and others). The upper extremity of the epimeron, extends the whole 
length of the mesosternal flank, terminating, at the upper articulation of the wing- 
case, behind the extremity of the episternum ; its upper edge is grooved to receive 
the margin of the inflexed base of the wing-case. 
There are some points specially characteristic of the mesothorax of the Dytiscidee, 
and a comparison of these with the Carabide is of interest; they are—l. The 
direction of the mesosternum; 2. The magnitude of the anterior sternal pieces ; 
3. The magnitude of the posterior piece ; 4. The separation of the middle coxe ; 
and 5. The penetration of the epimeron to the coxal cavity. 1. As regards the 
first of these, there does not exist to my knowledge any Carabid in which the 
direction of the mesosternum is so remarkably divergent from that of the meta- 
sternum as it is in most of the Dytiscidee, and in the great majority of the members 
of the two families this difference is very considerable ; there are, however, some 
Dytiscide—the Vatellini and Sternopriscus, (and even a species of the genus 
Hydroporus, Dytiscus dorsalis No. 630)—in which the mesosternum has only to a 
comparatively slight extent assumed the diaphragmatic position it possesses in 
their allies; and on the other hand in Systolosoma, Trachypachys and Cyclosomus 
of the Carabide, the direction of the mesosternum becomes to a great extent that 
of the Dytiscide ; no absolute distinction exists therefore between the two families 
in this respect. 2. The medisternum, and the episternum are very much larger in 
the Carabidee than they are in the Dytiscidee (the head and prothorax must be 
separated from the afterbody before this can be appreciated). This distinction is 
remarkable and is no doubt correlative with, if not dependent on the fact that the 
presternum has a connexion with the metasternum in the Dytiscidee, this being 
of course much facilitated by the abbreviation of the intervening mesosternum ; 
while as another explanation of the curtailment, we have the fact that the anterior 
part alluded to is prolonged to allow of a greater or less extent of rotation and 
nutation of the prothorax in the Carabide, while in the Dytiscide this power has 
been held in abeyance in consequence of the more supreme necessity for fixing and 
securing these parts to prevent the admission of water to the interior of the insect. 
Thus it is in the Carabide: where the head and prothorax are quite free and mobile 
that we find the greatest difference from the Dytiscide in the point alluded to, 
while where there is a prosternal process in the Carabidee the medisternum and 
episternum become shorter: Cyclosomus and Trachypachys, Systolosoma, with 
others of the fragmentary series of Carabide (in which series it is that we find a 
prosternal process of not unfrequent occurrence) have therefore shorter anterior 
