On Aquatic Carnivorous Coleoptera or Dytiscide. 959 
the antenna in Copelatus politus ; the display of a file of fine rugee in some of the 
species of Agabus (group 8) on the third ventral segment ; and the existence of a 
similar structure on the edge of the second ventral segment in numerous species of 
Colymbetes, and in a few Rhanti. The form of the coxal processes in Agabinus is 
very peculiar, reminding one somewhat of Noterus. Hydrotrupes and Coptotomus 
show peculiar labial palpi. And in Platambus we have a remarkable increase in 
the posterior portion of the epipleure, which however is approximated by certain 
- Agabi. In Matus we havea conspicuous sulcation of the prosternum which is repro- 
duced in Spencerhydrus, a genus of Cybistrini. While in several genera (Agametrus, 
Metronectes, and others) we have the very exceptional occurrence of a complete 
effacement of the coxal lines. Colymbetes reticulatus, (Lancetes) shows a difference 
in the colour of the ventral segments of the two sexes, similar to what exists in 
certain species of Bidessus and Coelambus, in the Hydroporides. 
The interesting Madeiran species of Meladema, (Dytiscus lanio) makes an approach 
to the genus Dytiscus in several respects, and it has the suture on the front of the 
head distinct as is the case in that genus, and the clypeus is coloured yellow as in 
that genus, while the two pale marks on its vertex seem to dicate the manner in 
which the angular mark on the front of the head of Dytiscus has been developed 
from the two pale spots so common in the Colymbetides. 
As regards geographical distribution little can be said, except that the great 
majority of Colymbetides are inhabitants of the northern parts of the Eastern and 
Western Hemispheres, but one large genus (Copelatus) is on the contrary chiefly 
tropical. 
Ill. 4—Tribe HYDATICIDES. (Vide p. 647.) 
This tertiary aggregate consists of about one hundred species arranged in five 
primary aggregates, plus three isolated species; and in the second synthesis these 
are reduced to two aggregates, plus an isolated primary aggregate. 
They are all insects of moderately large size—one-third of an inch to two-thirds 
of an inch in length: the colour of the surface is generally variegate. 
The head is short, the eyes are large, the insertion of the antenna is but little 
protected by the side of the front, and this does not infringe on the circular outline 
of the eye ; the frontal suture can be detected only at each side ; the antennze are 
elongate and slender but to a variable degree. The hind coxe are large, and the 
wing of the metasternum has always a slender sub-parallel deflexed band as its 
terminal portion ; this band is however very variable in its length according to the 
amount of forward extension of the hind coxa. 
The coxal lines are always short, (sometimes quite obliterated) and in front they 
are very far indeed from attaining the front border of the coxa ; the coxal lobes are 
never elongate and are always broad in proportion to their length, and when the 
