954 On Aquatic Carnivorous Coleoptera or Dytiscide. 
front and behind, and its posterior part projects as a fine edge, which laps over the 
basal sutural angles of the wing-cases when they are closed. 
In Hydroporus nigriceps (Hydroporus, No. 576) the anterior pieces of the 
mesonotum are smaller than in Dytiscus duodecimpustulatus ; the scutellum is 
however similar in form, but it is received into an angular depression at the base 
of the elytra, for the reception of the middle of the base of the prothorax, whereas 
in D. ducdecimpustulatus, the elytra are truncate at the base. 
In all the above insects the scutellum is completely concealed when the prothorax 
is in its natural position. In Hydroporus oblitus, however, a small portion of its 
apex remains uncovered, and in Celina it is largely visible. I have not been able 
to obtain a specimen of either of these insects for more detailed examination. 
Thus the definition “scutellum invisible” applied to the Hydroporides covers 
and includes a series of diverse and interesting modifications. 
The functional value of these parts is considerable. Nothing more perfect can 
be imagined than the manner in which the elytra when closed are locked together 
by different portions of their structure and by projections of the mesosterna and 
mesonota ; a complicated set of surfaces and edges is so beautifully coadapted that 
there cannot be the slightest admission of fluid from without, and great strength is 
attained although the material used is very small. 
Almost everything remains to be done in the study of these parts, for they have 
hitherto been but little examined. 
The Hydroporides are found in all parts of the world, except some of the Pacific 
islands. The Hydrovatini and Bidessini are widely distributed in the warmer parts 
of the world ; the Hyphidrini are more specially African ; and the Hydroporini 
Palearctic and Nearctic ; while Sternopriscus is peculiar to Australia, and Celina 
to the warmer parts of the New World. 
There exist in the Hydroporides numerous grades of development of various 
organs, and this instability of the structure, combined with the fact that some of 
the gradations can clearly be distinguished as higher or lower than others, suggests 
the idea that some of the species or genera may be placed in an ancestral relation 
to others. Hydrovatus, having very poorly developed locomotive organs, possesses 
a great resemblance to some of the species of Coelambus, such as Dytiscus 
ineequalis, and this latter form in the structure of its swimming legs might be con- 
sidered to be an advanced Hydrovatus, the apparent connection between the 
two being greatly increased by their possessing In common a margined head, and a 
genicular impression on the epipleura, as well as by the general form, by the 
sculpture, and by the structure of the antenne. Thus a truly genetic relationship 
seems to be suggested. But on continuing the comparison the probability of any 
such relationship quite disappears, for while in Hydrovatus the middle and hind 
coxze are widely separated and the prosterna] process of remarkable width and 
peculiar form, and the posterior coxal cavities covered and protected, in Dytiscus 
