On Aquatic Carnivorous Coleoptera or Dytiscide. 961 
shallow. In the structure of the hind legs Eretes shows some interesting pecu- 
harities, they are much more slender than in any of the other Hydaticides ; 
the femora especially are slender, the tarsi are elongate and their basal joint 
not very greatly shorter than the tibia: on the upper face of the femur close 
to its posterior edge is a band of pubescence, this band traverses the whole length 
of the limb, the pubescence at its commencement at the trochanter is quite short 
but gets longer as it nears the knee, and terminates there in a group of very 
elongate fine hairs; in Hydaticus there is usually an obscure band of very fine 
short pubescence, homologous to this band of Eretes, but placed at a distance from 
the hind margin; in other Hydaticini, this band is represented only by a series 
of isolated punctures (Dytiscus seminiger, &c.), and in others cannot be detected 
in any form : the tibia also shows some interesting modifications ; it is longer and 
more slender than in the Hydaticini and Thermonectini; the series of punctures 
on its upper face bearing elongate furcate setze is very highly developed, and is 
placed near the extremity (about five-sixths of the length from the knee), where it 
forms an obliquely transverse band, starting from the inner margin, and extending 
half way across the tibia; in the other Hydaticides a very different arrangement 
prevails, the furcate setee are either arranged as a series near to and parallel with 
the outer margin of the tibia, or as a transverse series on the middle of the face, 
but are never approximate to the inner margin or to the apex as in Eretes; it would 
appear, if this be so, that in the rudimentary stages of development of these furcate 
sete their position is as a series parallel with the outer margin, and Eretes is of 
all known Dytiscidze the one which has most widely departed from the primitive 
arrangement of these seta. 
The tibial spurs in the Thermonectini are bifid at the apex, this character is how- 
ever frequently so minute that it requires a careful examination to detect it ; these 
bifid tibial spurs, although wanting in Eretes and Hydaticini, reappear in 
Laceophilus and Cybistrini, and there this peculiarity of structure attains a more 
considerable development than in the Thermonectini. 
The straight episternal suture of the Hydaticini is an exceptional feature—a 
departure from the usual method of stucture of the part—for this suture is curvi- 
linear, not only in Thermonectini and Eretes, but also in the Colymbetides, 
Dytiscini and Cybistrini; in these groups however, the curvilinear suture is not so 
much curved as it isin the Thermonectini, so that as regards this structure we are 
brought face to face with the interesting fact, that the Hydaticini and Thermonectini 
are departures in opposite directions from the average or normal Dytiscid type. 
Such a consideration renders it difficult to imagine that the Hydaticides, in opposi- 
tion, say, to the Colymbetides, are descended from any common ancestor, but 
rather disposes us to believe, that both types of the episternal suture existed in the 
early ancestry of the Hydaticides, and that the distinctions between the two types 
would not then be so extreme as they are at present. This view is borne out by 
