926 On Aquatic Carnivorous Coleoptera or Dytiscide. 
development of the coxw is sometimes extreme; thus in Desmopachria the hind 
coxa almost reaches the middle articular cavity. 
In other respects very important differences exist between the various components 
of the aggregate. 
Tyndallhydrus possesses a prosternal process that does not connect with the 
metasternum, agreeing in this respect with the Vatellini, (of the Dytisci fragmentati,) 
and with Andex (of the Hyphydrini). 
In most of the species of Bidessus, the mesothoracic epimeron is very slender and 
even at its upper posterior (episternal) angle is so little enlarged that it may be 
almost described as linear ; on the other hand, in Pachydrus, it is comparatively 
well developed at the point just mentioned. 
In Desmopachria the parts of the prosternum are reduced to a minimum of size, 
forming -in fact merely a slender frame for the encasement of the front cox ; the 
anterior piece of the prosternum is almost as short as possible, the band between 
the coxe is extremely slender and the prosternal processis minute. In Huxelhydrus 
the prosternum is less reduced than in any of the other genera of the aggregate. 
In Bidessus, Huxelhydrus, (and probably in Tyndallhydrus), the mesosternal fork 
does not reach the intercoxal process of the metasternum, whereas in the other genera 
this connection between these two parts is well completed. 
Bidessus, Huxelhydrus, and Tyndallhydrus have the slender hind tibiz a little 
bent near the base, and becoming a little thicker from the base to the apex, 
whereas in the other genera the tibiz are stouter, and are straight, and of about 
one thickness froin base to apex. 
The three genera, Heterhydrus, Pachydrus, and Desmopachria, by several char- 
acters appear to form a naturally distinct aggregate from the other three, thus they 
are of peculiar short, broad, form, with a broad prosternal process, with thicker 
swimming legs, and straight tibize; short and compressed or subserrate antenne, 
and completely separated middle coxe ; and with contiguity between mesosternal 
fork and metasternal intercoxal process : whereas in the other genera, the form is 
more oblong, the prosternal process oblong, the middle coxze contiguous, and the 
mesosternal fork disconnected from the metasternal process, the hind tibice differently 
formed, and the antennz comparatively filiform. Although these characters are 
very considerable, yet the multitude of species included under Bidessus show much 
variety of form, so that it is possible that when carefully examined they may 
present intermediate forms. 
The Bidessini are distributed over all lands, but apparently prefer the warmer 
regions. ‘Tropical America seems to be the metropolis of the Pachydrus group of 
genera, Australia of the Bidessus group. No species has yet been found in the 
Pacific Islands. | 
I should remark that I have not been able to ascertain with certainty that the 
hind-body is actually soldered to the coxa in Heterhydrus and Tyndallhydrus, but I 
entertain almost no doubt that such is the case. 
