On Aquatic Carniworous Coleoptera or Dytiscide. 891 
The compression of the prosternal process is greater than inany group of Agabus 
except the 22nd group, 
I have not been able to observe the tomentum on the interior of the elytra 
in all the species, but I have found it trustworthy in all such cases as I have 
examined. 
Many of the species are very similar to one another, and require an examination 
before they can be distinguished with certainty. 
Considerable variety prevails in regard to nearly all the characters of the genus. 
The characteristic shape or form is scarcely present in certain species, especially 
the smaller ones: the lobing of the hind tarsi, very conspicuous in certain species, 
(Dytiscus ater, e. g.,) is much less developed in others (I. oblitus for example). 
The swimming legs are better developed in I. apicalis than in any other species, and 
they are in it much thicker, shorter and more powerful than they are in I. disce- 
dens ; these two species being the extremes of the genus in this respect. The 
hind coxze and the wings of the metasternum, also show considerable variety, in 
fact there are scarcely two species which agree exactly in the size and form of thes? 
parts. 
The external sexual differences are confined to the legs and last ventral segment; 
difference in sculpture between the two sexes being quite absent; the male tarsi are 
never highly developed, the incrassation of their joints being at most only moderate; 
the clothing of their undersurface however attains a considerable length in some 
species (cf. Dytiscus ater), and as it spreads out in a divergent manner increases 
very much the apparent size of the tarsus; these hairs bear very small paper-like 
palettes at their extremity. The last ventral segment is usually coarsely strigose 
in the male (though to a variable extent) on its apical portion, and very frequently 
has a fine carina on the middle of this part; in the female this segment has its 
apical portion compressed on each side in a peculiar manner, so as to form a notch 
or groove which varies in apparent shape in accordance with the point from which 
it is viewed. 
The genus is characteristic of the northern portions of the two hemispheres, and 
there is no doubt, that the north of America will prove to be its metropolis; I. 
discedens must be considered the most primitive of the species yet known ; it has 
poor swimming legs with the set at the outer angle of the femur very small, 
rather small coxze, feeble development of the male feet and their clothing, the 
claws of its hind tarsi are less differentiated than in other species, the lobing of 
the joints of the hind tarsi is small, and the characteristic facies of the genus is but 
little developed init. The few species known from the eastern portions of the 
Old World and Japan are aberrant, as is also the I. cinctus of Thibet. 
I have arranged the species in two groups according to whether the hind tarsi 
of the males have on the outside the joints margined beneath, or quite unmargined. 
TRANS, ROY. DUB. SOC., N.S., VOL. LU. 5 YX 
