218 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



process ; this process differs greatly in its shape or form, and offers very important 



and constant means of recognizing some of the genera and species : it is sometimes 



very broad and shoi-t, the extreme of this condition being seen in Hydro vatus, 



where the prosternal process projects but little behind the coxte, and is much 



broader than long, and is moreover nearly truncate behind, the hind margin 



showing however (when the prothorax is separated from the after-body) an obscure 



angle in the middle ; the other extreme is seen in Laccophilus, where the prosternal 



process, is very narrow, and is prolonged backwaids as a longer or shorter slender 



spine ; a great number of other forms more or less intermediate exist between these 



two extremes ; but in Neptosternus we find a very exceptional prosternal process, 



the process itself being trisjMuose, and consisting of an elongate slender spine in 



the middle, with a shortei', and very slender, divergent spine on each side. The 



prosternal process is either flat in the transverse direction, as in most Hydroporides, 



some Agabini (Platambus, &c.), or is more or less compressed (Ilybius), convex, 



(Coptotomus) or even indistinctly carinate along the middle (Herophydrus) ; it is 



margined at the sides, the margin being the backward prolongation of a similar 



margin existing along the front of the coxal cavity {vide Cybister) ; this margin 



sometimes extends for the whole length of the process {vide many Agabini, Dytiscus 



fuscipennis No. 752 e.g.), or becomes slender and terminates before the extremity 



(Dytiscus, Cybister and many others) ; sometimes the margin exists at the side of 



the prosternal process, although it has become quite obsolete along the front of the 



coxal cavity, and it appears to be the rule in th3 Hydroporides that this margin of 



the coxal cavity and prosternal process is absent or only very partially developed : 



the most remarkable condition of the margin of the prosternal process is that found 



in Neptosternus, for on examining N. tridens in comparison with some other 



Dytiscidse, it will readily be seen that the lateral spines of the prosternal process 



are simply the lateral margins which are detached and divergent ; this peculiar 



condition of the prosternal process in Neptosternus is well worthy of the attention 



of entomologists who think that the doctrine of community of descent may be true 



in the case of allied genera and species, if not in the case of all living beings ; for 



we have here a perfectly isolated and most peculiar form of appendage, which there 



seems every reason to believe must have been developed pari passu with the 



prosternal process, for it is almost impossible to suppose that the margin was in the 



ancestors part of the prosternal jarocess, and has since become gradually detached; 



whereas we can well understand the prosternal process and its margins as being 



each a distinct growth, amalgamated in other beetles, but distinct in Neptosternus ; 



if such be the case how far back must we go in the ancestral record before we 



could hope to find a common ancestor for the allied genera Neptosternus and 



Laccophilus ? We must go back to the period when there existed no prosternal 



process. But the existence of a prosternal process is an absolutely constant feature 



of the Dytiscidse ; and must no doubt have been one of the earliest developed of 



