904 On Aquatic Carnivorous Coleoptera or Dytiscida. 
The swimming legs and hind coxz are quite as well developed as they are in the 
most perfect species of Dytiscus. The form of the coxal lines and processes is very 
similar to that of Dytiscus hybridus. 
We find in Hyderodes, as in Dytiscus, dimorphic females; certain individuals 
of that sex bemg smooth like the males, while others have the thorax and elytra 
roughened, by a peculiar, very coarse and deep, irregular sculpture, giving rise to 
a corroded appearance. 
I. 66.—Genus DYTISCUS. (Vide p. 634.) 
All the species forming this aggregate (twenty-two in number) are of large size, 
(an inch or an inch and a half long), the upper surface is of dark colour with a 
yellow stripe along the side of the thorax and elytra, the clypeus is yellow, and there 
is on the middle of the head an angular yellow mark; in addition to these yellow 
marks some species have the anterior and posterior margins of the thorax yellow, 
and the eyes margined with yellowish colour. The colour of the under surface is 
either pitchy black or yellow, or is intermediate between the two colours, or a 
mixture of them. The form is comparatively little convex, always elongate, but of 
variable width. 
The clypeus is always separated from the front of the head by a suture visible 
across the whole width of the head. The prothorax is destitute of a lateral margin. 
The prosternal process is of variable length, usually rather elongate and narrow, 
being only very little widened out after passing the coxz, it is not compressed, and 
is indistinctly margined at the sides, the margin not extending to the extremity ; 
in a few species the process is short and comparatively little acuminate (¢.g., D. 
latissimus, and D. lapponicus). The inter-coxal process of the metasternum is 
occupied by an elongate narrow depression for the reception of the prosternal 
process. The hind legs are but little developed for swimming, being elongate and 
rather slender ; the femur has a group of accumulated setigerous punctures at the 
extremity that does not quite extend to the hind margin; their tibie are usually 
about three times as long as they are broad ; the tarsi are considerably longer than 
the tibia, and are terminated by two rather slender, curved, nearly equal claws, the 
hind margins of their joints are but little lobed externally. In the more perfect 
species (¢.g., D. hybridus, and habilis) the swimming legs have become shorter and 
thicker, and their claws more unequal. 
The hind coxee are rather small, and their anterior border is not much arched, 
the wings of the metasternum are only of moderate area. ‘Phe coxal lines are but 
little bent, and the coxal lobes have a great extension in the longitudinal direction, 
while they are comparatively small in the transverse direction ; the coxal border is 
elongate, and before the apex is usually a good deal broader than in front, the 
coxal notch is elongate, and beyond it the processes tend, in numerous species, to 
lengthen and become slender, so as to form two spinose projections in the extreme 
