On Aquatic Carnivorous Coleoptera or Dytiscide. 935 
group Dytiscini), so that the character is of considerable taxonomic importance, and 
will probably be found to have a considerable functional value in the respiratory pro- 
cesses. I have examined a great number of species in addition to those I have actually 
dissected, and I have not detected any trace of the existence of these rugee, even in 
a rudimentary form, in any other Dytiscide. 
The setigerous punctures on the hind femur are variable in the group, but they 
never assume the form which they constantly present in the Agabini. In some 
species they are entirely absent, Dytiscus calidus (No. 942), for example, while in a 
few other species of Rhantus (Dytiscus notatus, e.g.) and in most Colymbetes they 
are rather numerous and conspicuous, and all the intermediate grades occur ; on 
the whole it seems that in the Colymbetini the tendency is for these setigerous 
punctures to disappear in the higher forms, while, on the contrary, in the Agabini 
it is in the higher species that they display their greatest development and are most 
conspicuous. 
Three of the four genera forming the group agree in possessing the ventral side 
pieces narrow, and are thus sharply distinguished from the Agabini; the fourth 
genus (Scutopterus) however departs abruptly from the other genera to agree in 
this respect with the: Agabini. In Colymbetes and Meladema, the width of the 
ventral side pieces is so much reduced that the fourth may be described as linear, 
its length being six or eight times its breadth. In Rhantus the reduction of width of 
this piece is not so conspicuous as it is in the aggregates just mentioned, but still it is 
so great that the genus is by that character alone perfectly distinct from the 
Agabini ; in Col. grapii, which so far as I have observed is the species of Rhantus 
that comes nearest to the Agabini in this respect, the length of the fourth side 
piece is four or five times its width; while in Agabini (as mentioned in that group, 
p. 932) the length is only about twice the width. Scutopterus however forms a 
striking exception to the other genera of Colymbetini, the length being only about 
twice the width. 
Another character by which the Colymbetini is contrasted with the Agabini is 
the greater development in the former group of the penultimate ventral stigma. 
There is however much variation in this respect, and it is not necessary to go into 
details of the variations, it is sufficient to remark that in this respect it is not Scu- 
topterus that most approaches the Agabini, but Rhantus. In Colymbetes this 
stigma is transversely elongate, while in some of the smaller Rhanti (Dytiscus 
exoletus, No. 951, e.g.) it remains smaller than it is in some of the Agabini. 
The hind tarsi show very frequently a lobing of the posterior margins of the 
hind tarsi externally, but to a very variable extent. In Colymbetes this lobing 
is extreme, while in Rhantus it is variable and in some species (Col. binotatus, No. 
935, e.g.) is but slight. 
The claws of the hind tarsi are usually very unequal in the Colymbetini, and 
are nearly straight, the inner one being very large, and twice or three times as long 
