On Aquatic Carnivorous Coleoptera or Dytiscidce. 831 



record, when they were in a stage of evolutiou more primitive than that of any 

 Carabidife now remaining to us. Embryological studies may here give us 

 important assistance. 



The following brief recapitulation will show the justice of this assertion. It has 

 already been shown by Schaum and others that Pelobius has the head of Carabidse, 

 besides this its antennae approach more nearly to those of the Carabidse than to 

 those of any Dytiscidfe, except the spurious Dytiscid, Amphizoa : the prothorax is 

 quite that of Carabidse, except that the prosternal process connects with the metas- 

 ternum and even this character is not foreign to Carabidas. (Compare Cyclosomus). 



The mesosternum is absolutely that of one division of Carabidae. 



The metasternum is that of Carabidse, and possesses behind the transverse stituie 

 that exists in the majority of Carabidse, but in no Dytiscidse, except the spurious 

 Dytiscid, Amphizoa. 



The hind coxse are those of Carabid^, with some slight modifications, viz., that 

 they are a little increased in size, that their internal laminse are accurately co- 

 adapted (instead of touching only at one point), and that they are marked by an elon- 

 gate excision for the play of the swimming leg, permitting cf a greater movement of 

 the trochanter in one direction. 



The legs are those of Carabidse except that they bear cilise. 



The genera] form is foreign to Dytiscidse, for it is tub-like and does not show that 

 evenness of outline conspicuous in the water beetles ; the sculpture too is that of 

 Carabidse rather than that of Dytiscidse. 



It seems clear then that Pelobius cannot be satisfactorily classed with the 

 Dytiscidse ; and it is equally clear that it is a Carabideous insect havino- consider- 

 able modifications to adapt it to move in water. 



It is therefore only included provisionally in my classification of Dytiscidse. 



Although I think Pelobius may be admitted among the Carabidaj, it will be there 

 an absolutely isolated form. In the present state of knowledge of organic nature 

 no animal having any affinity whatever to these three species can be pointed out. 



The isolation of Pelobius and the geographical distribution of its component three 

 species are in themselves facts of much interest, for it is almost impossible to suppose 

 that there can have been developed both in Europe and Australia absolutely ah 

 initio animals which, notwithstanding their widely different environment have 

 evoluted into a similar form which is absolutely isolated from all other known forms. 

 It is almost impossible to suppose this, and we cannot but believe that the European 

 species and the Australian species lived during the greater part of their ancestral 

 record side by side, and only became separated from one another when their present 

 structure was comparatively nearly established. It being then quite probable that 

 the Austi'alian species and the European species formerly had a common habitat, it 

 will be an interesting question to consider at what period of the world's history this 

 could have been the case. Huxley has already suggested an answer to this question 



