On Aquatic Carnivorous Coleoptera or Dytiscide. 941 
tibial spurs is present in no member of the Hydativides except the Thermonectini. 
The most aberrant member of the aggregate is Acilius, and it is quite possible that 
this may ultimately have to be separated ina more marked manner from the allies : 
but Dytiscus mediatus, No. 1057 (which however I have only been able to study 
in an incomplete manner), seems really to connect the other species of Acilius 
with Thermonectes. 
Thermonectini are found in all the warmer parts of the globe; two of the genera 
however (Acilius and Graphoderes) occur only in the temperate parts of the 
northern hemisphere. 
II. 15.—Group Cysistrini. (Vide p. 700.) 
This secondary aggregate or group consists of four genera comprising seventy-two 
species. The size is usually large, but varies much (from 14 m.m. to 48 m.m. of 
length). The outline is very perfect and continuous, and punctuation is absent, so 
that the surface is polished and smooth, except that the females of many species 
show a sexual sculpture on the upper surface consisting of short lines or scratches. 
The swimming legs are very powerful, their tibiee are very short and broad, and 
their apical spurs very unequal, the inferior one being very broad, and also longer 
than the other. The front tarsi of the males have the three basal joints greatly, 
frequently enormously, dilated in the transverse direction, and furnished on the 
underside with a large number of palettes, all quite similar to one another, and 
forming four crowded series; the large disc, formed by the dilatation of the three 
joints alluded to, is fringed along its margins with fringing hairs, and at the base or 
heel, between the basal row of palettes and the fringing hairs, there is an area of 
variable size, occupied by dense fine hairs or setze (pubescent area). The inter- 
mediate male tarsi are without any dilatation or thickening of the basal joints in 
the transverse direction. When there is a sexual sculpture present in the females, 
it consists of fine scratches or short lines, which may extend over elytra, thorax, 
and head, or some part, or parts thereof. 
The characters above enumerated are present in all the Cybistrini, and are quite 
sufficient to validate the group; but it is also distinguished by numerous other 
positive and negative characters, not quite so definite or constant as the preceding 
ones. 
The two terminal joints of the male front tarsi and their claws are but little 
elongate, and the palettes of their undersurface are peculiar in their structure, each 
being oblong in form, and the outer portion of a peculiar thin or papery-like nature, 
while the basal portion of each is more solid and is deeply impressed or pitted; the 
stalk by which the palette is connected with the tarsus is attached to this more 
solid basal portion: (the genus Spencerhydrus forms however an exception to the 
other Cybistrini in respect of the structure of the male feet, the terminal joints of 
6 E2 
