On Aquatic Carnivorous Coleoptera or Dytiscidce. 859 



no near allies, and is an exemplification of the peculiar isolation so frequently found 

 to exist in tlie 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, in 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 coxse 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 Avith any nearness, and I have 

 decided that it is the more natural course to le&^e 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 IIYPHOPORUS. [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 I'oint 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 epipleura3 are very slender in their posterior portion ; the articular 

 cavities of the hind coxse 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 a short, rather broad, coxal border. 



The genus as above stated approximates to Hyphydrus, but its closest ally is 



TEANS ROT. DUB, SOC., N.S., VOL. II. ^ 



