On Aquatic Carnivorous Coleoptera or Dytiscide. 859 
no near allies, and is an exemplification of the peculiar isolation so frequently found 
to exist in the case of these so-called synthetic types. In general appearance and 
in the structure of the mesosternal parts, Huxelhydrus of the Bidessini makes a 
quite incontrovertible approach to Sternopriscus ; while from another direction 
Necterosoma of the Hydroporini approximates it by the 5-jointed tarsi; and even 
Dytiscus dorsalis in Hydroporus makes some approach to it, in the mesosternal 
structure and some other points. 
Sternopriscus is peculiar to Australia; and it may here be mentioned that 
Huxelhydrus and Necterosoma just alluded to are also exclusively Australian ; we 
are, Im my opinion, not justified from these facts in inferring a genetic connection 
between these forms, but I think we are quite justified in concluding that the 
conditions of existence of certain Hydroporini in Australia have induced the 
development of similar structures in perfectly distinct creatures. The similarity of 
structure is evidence not of descent from a common ancestor, but of conformity of 
the conditions of existence at certain periods of the ancestral records. 
The classification of Sternopriscus is very difficult, for although it is clear that it 
is correctly placed amongst the Hydroporides, it does not appear possible to locate 
it satisfactorily in any of the four secondary aggregates which form that group ; if 
we take the aggregate of its characters it would appear to be nearest to Huxelhydrus 
in the Bidessini, but it does not possess the soldered coxze and ventral segments, 
which is the essential character of that group. On the other hand there is no 
member of the Hydroporini to which it approaches with any nearness, and I have 
decided that it is the more natural course to leave it at present isolated during the 
second synthesis, and to unite it with other components to form the Hydroporides 
of the third synthesis. 
I. 32.—Genus HY PHOPORUS. (Vide p. 390.) 
Three species form this aggregate, their individuals have the same appearance as 
the species of Hyphydrus, to which they are similar in size, form, and colour: and 
the similarity of structure of the front tarsi, in combination with these slighter 
characters, indicates an undoubted approximation between the two genera; the third 
joint of the front tarsus is large, and little emarginate, and the terminal joint short 
and little exserted. The front of the head is rounded and margined. The elytra 
have on the shoulder of the epipleura a definite genicular fossa, which is limited 
externally by a raised line, and the ligula on the inner face of the elytra is highly 
developed ; the epipleurx are very slender in their posterior portion ; the articular 
cavities of the hind coxe are slightly but quite distinctly separated, and the extremity 
of the upper covers of the cavities is distinctly displayed beyond the apices of the 
coxal processes ; the coxal lines are distinctly sinuate or turned outwards near their 
apex, and there is ashort, rather broad, coxal border. 
The genus as above stated approximates to Hyphydrus, but its closest ally is 
TRANS ROY. DUB, SOC., N.S., VOL. II. 5 § 
