236 On Aquatic Carnivorous Coleoptera or Dytiscide. 
point of connexion form a very acute angle, and proceeding forwards like the 
branches of a letter V diverge but little till they have become quite near to the 
middle coxa when they are rather abruptly turned outwards. In Hydrovatus these 
sutures of the two sides diverge from one another only at a very obtuse angle, and 
therefore are directed but little forwards at first, but when rather near to the out- 
side of the body, the direction becomes more abruptly forwards, but only to be 
almost immediately even more abruptly bent back : a similar form of the coxa 1s 
pretty general in the Hydroporini. The anterior border of the coxa of the 
Dytiscide thus forms in front an arch which is very various in its form, in Eretes 
as we have seen it is a broad, very flat arch; while in Hydrovatus a narrow abrupt 
curve placed near the outside of the body is formed. Various intermediates 
between these extremely different forms occur ; and it is only in the Noterides that 
we find a really distinct form of the coxa, owing to the fact that this part attains 
its greatest extension forwards near to the middle of the body, and then becomes 
shorter towards the side. In the Cybistrini the coxa assumes in front a shape 
characteristic of the group, its greatest anterior extension being gained near the 
outside of the body, while external to this point it so abruptly retreats as to form 
almost a right angle. In size the external lamina varies even more than it does 
in form; it is, compared with the average of other Coleoptera, always large, and 
within the bounds of the family is least in Pelobius and Amphizoa; in certain 
species of Agabus (Dytiscus uliginosus, No. 694, Agabus maderensis No. 666), it 
is not greatly larger than in Amphizoa, and with every gradation of growth in 
various species and genera, reaches a truly enormous size in Hyphydrus, Hretes, 
Acilius and Coptotomus. 
Though the external lamina is of much interest and importance, yet the internal 
lamina on account of its being the seat of the articulation of the portion of the leg 
used for swimming is much more important, and much more complex and varied. 
The base of each trochanter is placed in a kind of box, and the larger the opening 
of the box the greater is the range of motion of the limb, and the more powerful 
its sweep, and in the higher forms of the Dytiscide the articulation is constructed 
so as to allow the leg to make a sweep extending round the complete half cireum- 
ference of a circle, and yet the articulation is so well constructed that no water 
can obtain entrance by it to the interior of the body where the muscles are situated. 
The socket is formed above by an arched plate, which appears to be merely a 
prolongation of the abdominal portion of the external lamina of the coxa, and 
below by the coxal lobe or process, there being left between these two parts a 
large circularly transverse cleft looking backwards and laterally ; the internal wall 
is concealed owing to its being closely connected with its fellow of the opposite 
side of the body ; in fact in all the higher forms of the family, there seems to be 
only a single opening for the two legs—in other words the two transverse 
clefts appear joined into one—but when a dissection is made, it is seen that they 
