218 On Aquatic Carnivorous Coleoptera or Dytiscide. 
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 short, the extreme of this condition being seen in Hydrovatus, 
where the prosternal process projects but little behind the coxe, 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 backwards 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 trispinose, and consisting of an elongate slender spine in 
the middle, with a shorter, 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 (wide 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 the 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 
Dytiscidee, 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 process, 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 Dytiscidee; and must no doubt have been one of the earliest developed of 
