962 On Aquatic Carnivorous Coleopfera or Dytiscidce. 



Prodaticus pictus, which is the most primitive of existing Hydaticini ; in it the 

 episternal suture makes a considerable approximation to the average Dytiscid 

 type, as displayed by, say, Colymbetes pulverosus, but still it shows the peculiarity 

 characteristic of the Hydaticini markedly displayed ; so if w^e go to the Thermo- 

 nectini and take their most primitive existing form — Acilius — we find the departure 

 from the average type of episternal suture perfectly marked in the opposite direc- 

 tion from Hydaticini, though to a much less extent than it is in the higher 

 Thermonectini — Thermonectes for example. Thus, the conformity of structure of 

 the hind tarsi of the Hydaticides, though it justifies us in classifying them 

 together, does not justify us in considering that they are descended from a 

 common ancestor, qua episternal suture. If we now turn to the Hydaticides 

 which have the episternal suture of similar structure, viz., Eretes and 

 Thermonectini, we shall find that this similarity of structure does not warrant 

 any belief in descent from a common ancestor, although at first such a descent is 

 readily suggested, and appears to be confirmed by other peculiarities. Thus for 

 instance, Thermonectes approximates Eretes, not only by the condition of the 

 episternal suture, but by the diminished epipleurse, by the large eyes, by the 

 exposed apex of the metathoracic epimeron, and the small terminal stigmata ; they 

 thus appear to be bound together, and these points would be cited as evidence of 

 descent from a common ancestor, and yet in other respects, Eretes and Thermonectes 

 thus bound together must be widely separated, and placed on opposite sides of a 

 hypothetical common ancestor ; for example, Eretes has a wonderfully developed 

 band of pubescence on the upper face of the hind femur, and of this there is no 

 trace in Thermonectes, although in the more distant Hydaticus the band is present 

 in a rudimentary form ; the tibial spurs too are bifid in Thermonectes, simple in 

 Eretes and Hydaticus. If we carry this analysis to other points of structure, a 

 similar inconsistency is repeated, and the search for a common ancestor or the 

 endeavour to construct such by imagination becomes more and more bewildering 

 and unsuccessful, in proportion as the facts considered become more extensive and 

 exhaustive. 



The tribe is widely distributed over the earth's surface, but does not occur in 

 New Zealand and the Pacific islands. 



