946 On Aquatic Carnivorous Coleoptera or Dytiscida. 
episternum, and very small in width at the posterior angle, which itself is obtuse and 
little prominent. This though true of all Noterides is not untrue of all other 
Dytiscidze, for such a description applies aiso to Huxelhydrus of the Bidessini. The 
fork of the mesosternum always is highly developed, and accurately coadapted with 
the inter-coxal process of the metasternum, and one of the most striking charac- 
teristics of the Noterides is the accurate coadaptation of the three parts, prosternal 
process, mesosternal fork, and inter-coxal process of metasternum ; it is by this that 
the immobility of the prothorax is secured; and the perfect continuity of the 
prothorax and after body is thus rendered possible. 
The middle legs in the higher Noterides are a good deal thickened and flattened 
out (see especially Synchortus), and their coxze are small and globular ; they play 
no doubt, as above observed, a considerable part in the locomotion of the individual, 
and have undergone a considerable change to facilitate this. This is quite in 
opposition to the higher forms of other groups of Dytiscide, for in them the middle 
legs do not share in a correlative manner the development of the hind legs into 
swimming organs. 
The metasternum is peculiarly formed in the higher Noterides ; its middle part 
is prominent, and very closely soldered with the prominent coxal processes, so that 
the suture is more or less nearly obliterated ; it is short in the middle, and forms 
only a very obtuse point there ; extending laterally this suture has very little 
anterior direction, but owing to the brevity of the metasternum, it is never far 
separated from the middle coxa, and sometimes indeed almost touches the coxal 
cavity near its middle (vide Noterus), it then proceeds almost directly outwards 
with a slight slope in the backward direction till it attains the episternum ; its 
anterior part in the middle always forms a very distinct and rather broad inter-coxal 
process, and on the outside of the coxal cavity extends a good deal forwards; a 
lateral wing of the metasternum is thus formed, having a very peculiar shape, this 
shape depends on the middle coxa having, as it were, by the unusual growth of the 
mesosternum, been forced backwards so that its cavity intrudes greatly on the 
metasternum, and as the metasternum is short, and has suffered compression from 
behind in the middle, by the growth of the peculiar and enormously developed coxal 
processes, the consequence is that on the space between these two forces it has 
become very reduced, and the lateral wing is almost cut off from the middle part. 
This peculiar structure is best studied in Noterus (vide fig. 39) and Synchortus, but 
where the swimming powers of the middle and hind legs are but little developed, 
as in the Suphisini, all that can be said is that there is such a form of the parts as 
might be developed into the peculiar structure described, by such changes as are 
necessary to improve the swimming powers. Thus in Suphis difformis the middle 
coxa intrudes comparatively little on the metasternum, and the hind femur when 
flexed for the stroke of propulsion can be brought comparatively little towards the 
middle, so that its stroke is much less powerful than it is in Noterus and 
