222 On Aquatic Carnivorous Coleoptera or Dytiscide. 
Thus in the very different Pelobius, Amphizoa, Dytiscidee, and Haliplide, we find 
a similarity of structure of the prothorax existing, not because of community of 
descent, but because water having to be excluded from the interior of the body, the 
simplest and best method of accomplishing that end has beenused in all these different 
forms. As regards the peculiar Carabide, Trachypachys and Systolosoma, we do 
not know enough of their mode of life to explain the structure: but as regards 
Silphomorpha and their allies we know that they are dwellers under bark, and we 
can feel sure that if this has been the case throughout a very long period of the 
ancestral record it must evolve a structural condition very different from that of 
the cursorial Carabide. Omophron has a peculiar mode of life, concealing itself 
for long periods in fine wet sand, and we find that in it the articulation between 
the pro- and meso-thoraces is well fitted to guard against the entry of fine sand, 
although it would not be sufficient to keep water from gaining access to the interior. 
The prothoraciec stigma is placed high up, near the junction of the epimeron with 
the pronotum, and is protected by a hollow in the front edge of the mesothoracic 
episternum ; the orifice is short and small, but the stigma is perfectly fitted for 
respiration, although it is not used for inspiration during the aquatic life of its 
possessor. 
The Mrsornorax in the Dytiscidee might, when the under surface of the body is 
looked at, be supposed to be absent, for it is not exposed in front of the metasternum, 
but is, asit were, turned inwards, so as to be placed more or less at right angles with 
the longitudinal axis of the body; the result of this is, that a kind of cavity or 
fissure, roofed above by the sides of the prothorax, exists in front of the metasternum, 
and in this hollow the flexed front and middle legs are packed during the process 
of swimming. This position of the mesosternum is however by no means invariable 
in the family, for though carried to an extreme extent in Hyphydrus, Hydrovatus, 
Cybister, and other higher forms, yet in the Vatellini, Sternopriscus and others the 
position differs but little from that of the Carabidee. The mesonotum has been hither- 
to but little studied although the visibility or invisibility of the scutellum has largely 
determined the classification of the family. It offers however considerable modifica- 
tions which should not be neglected. The scutum appears in the form of two 
lateral lobes joined together along the middle line; their base is nearly straight 
except that in the middle it is frequently (Dytiscus, Cybister, Graphoderes, &c.) 
intruded on by an angular projection from the middle of the front of the scutellum ; 
taken together the two lobes appear in front always more or less emarginate, being 
longest at the outer side, shortest at their point of junction in the mesial line ; this 
emargination may be very slight (Dytiscus bipustulatus No. 751 gen. Agabus ¢.9.,) 
or very deep (Dytiscus reeselii No. 1169, Cybister e.g.) In Noterus the base of the 
scutum is deeply transversely impressed, and the impression divided in the middle by 
a raised line, but in general the lobes are slightly convex, and without any definite 
impression. The mesonotum is very small in Hyphydrus, Laccophilus and 
