On Aquatic Carnworous Coleoptera or Dytiscide. 927 
Il. 8.—Group Hyrniprint. (Vide p. 370.) 
About thirty species, of which no less than twenty-four form the genus 
Hyphydrus, while the others are all autogenera, form this secondary aggregate. 
The size of the individuals is always small but never minute, the greatest length 
being 6 m.in., the least 3 or 4 m.m. In respect of form, colour and sculpture there 
is however much variety. ‘The positive characters distinctive of the group are 
found in the hind coxe and their articular cavities ; these latter are never contiguous, 
and are quite exposed, being unprotected by the coxal processes ; these are adpressed 
to the level of the ventral segments and thus separate the coxal cavities, their 
postero-external angles are obtuse ; there is no coxal border, and the coxal lines 
are not curved outwards near the extremity. The hind coxee themselves have an 
extreme development, and extend forwards so as to reduce the side-wing of the 
metasternum to a very slender band. All the components moreover have a largely 
developed prominent ligula on the inner face of the elytra near the apex. The hind 
coxa is free and not soldered to the ventral segments. 
Although these insects appear to be approximated by the Pachydrini, by virtue 
of the form of the hind coxee and their exposed articular cavities, yet they remain 
very distinct by the unsoldered coxee. In Pachydrini, where the adpressed coxal 
processes are soldered with the ventral segment, the process of evolution can only 
bringabout approximation of the trochanters by diminishing the intervening coxal pro- 
cesses, but in Hyphydrini where the processes are not soldered to the ventral segment, 
approximation of the trochanters becomes possible, by means of freeing the extremity 
of the coxal process from the level of the ventral segments, and the playing of the 
trochanter above the liberated portion if extension inwards of the articular cavity 
accompanies the change of level of the process; now if Hyphydrus major be ex- 
amined it is seen that the outer angle of the coxal process is really somewhat 
detached so as to form as it were the beginning of such a process of evolution. * 
Asregards the structure of the articulations of the swimming legs, the Hyphydrini 
must be I think regarded as an imperfect form, the coxal processes serve in no 
respect to protect the articular cavities, and the pyxal processes project beyond the 
coxal processes : but in other respects the group has attained a high degree of evolu- 
tion. The hind coxe are extremely large, and in Hyphydrus have actually attained 
about the maximum of possible extension. There are no forms known in which the 
cox are small, and the lower forms of the aggregate are in this respect very 
highly developed. We may I think conclude that we have here then highly evoluted 
forms of an inferior type of structure; and also that the surviving species have 
been able to maintain their existence by means of extreme evolution of certain 
* The process of evolution in the Hyphidrini may actually have been the reverse of what is sketched 
above, this being one of the cases where we can readily see the transitions that may have occurred, but 
not so readily determine which marks the starting, which the terminal point of the series. 
