On Aquatic Carnivorous Coleoptera or Dytiscide. 229 
lower level than it, it extends completely across the body as a horny band con- 
necting by its extremity with the metathoracic epimeron ; to its hind margin is 
attached the basal abdominal segment. In Hydrocanthus australasie this band is 
very peculiar, for instead of passing across the body in a straight line, it diverges 
backwards in the middle, so that between the scutellum and post-scutellum there 
exists a large membranous space. 
The metasternum consists of two side pieces on each side, and a middle piece— 
the metasternum proper ; this last is of very peculiar form, its hind margin 1s very 
firmly connected with the front of the hind cox, so that these are frequently 
supposed to be a portion of the metasternum ; in the middle in front this latter 
stretches forward and extends between the middle coxze, so forming an intercoxal 
process ; on each side of this are the middle coxe ; an inflexed portion of the 
metasternum forms in fact the posterior half of the coxal cavity ; outside the coxze 
the metasternum comes in contact with its episternum, and forms a curved or oblique 
suture in adaptation with this part; its hind border is encroached on by the exten- 
sion forward of the hind coxee, and thus between the episternum and the coxa, the 
metasternum is more or less—frequently excessively abbreviated—so as to form a 
wing, called the lacinia or wing of the metasternum ; in the posterior direction the 
metasternum projects farther back at the expense of the coxee, so forming in the 
middle behind a more or less acute angle ; thus the metasternum is elongate along 
its middle, but extremely short externally on each side. 
The intercoxal process in the majority of the Dytiscide is connected in front with 
the fork of the metasternum, and when so connected it is marked in the prominent 
middle part with a groove or depression —the metasternal groove—for the reception 
of the apex of the prosternal process, which is usually lodged in it: the intercoxal 
process differs much in shape and in its various details, and is one of the best means 
of recognizing genera and species: when the mesosternal fork is not sufficiently 
elongate to reach the same plane as the front of the metasternum, then the apex of 
the metasternal process is curved or bent up, so as to enter the mesosternal fork 
(Dytiscus) or at least to touch it (Hyphydrus, Eretes) ; the nature of the intercoxal 
process seems chiefly to have been determined by the distance separating the 
middle coxee ; when the middle coxe are quite contiguous, and project from their 
cavities, then there is properly speaking no intercoxal process (Vatellini, Sterno- 
priscus) the middle of the metasternum showing merely a prominent angle behind, 
and in adaptation to the coxte ; where the middle coxe are broadly separated (as 
in (Hydrovatini, Pachydrus) then the intercoxal process is correspondingly broad 
and short, and its connexion with the mesosternal fork is very intimate or exact, 
thus in Pachydrus each of its lateral angles projects and articulates with the raised 
angle of the extremity of the mesosternal fork, which in these cases is reduced, in 
the longitudinal direction, to a mere lamina, while the raised margin round the 
coxal cavity is continued inwards along the front of the intercoxal process; in the 
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