On Aquatic Carnivorous Coleoptera or Dytiscida. 851 
Middle cox rather widely separated ; the intercoxal process of the metasternum 
is slightly emarginate in front, in adaptation to the prosternal process ; the metaster- 
num is elongate in the middle, and its side wings are extremely slender ; the hind 
coxee are extremely large, their articular cavities are very widely separated, and 
are quite exposed and unprotected, being placed quite outside of the broad coxal 
processes which are adpressed to the level of the ventral segments ; the hind border 
of the cox is completely soldered to, and amalgamated with the basal ventral 
seoment. The hind legs are rather slender, their tibize and tarsi not much ciliate, 
the latter terminated by a stout short claw, (there exists a second claw, but it is so 
short that it can only be detected by a most careful examination). On the inner 
face of the wing-case the raised ridge has a very strong double wave near the 
extremity. The anterior and middle tarsiare small, four-jointed, the third joint 
bilobed, the fourth joint exserted, and of about the same length as each of the other 
joints. 
Although these insects have hitherto been classed with the old world Hyphydri, 
and not in any way distinguished therefrom, they are really widely different. The 
soldered ventral segment is of great importance as a distinguishing character, but 
besides that, the genus has the hind articular cavities more widely separate, and 
the upper border of the coxe different in form; the prosternum is still more 
reduced, while its process is broader and larger, and the inter-coxal band is prominent; 
the anterior tarsi are formed as in Hydrovatus, and very different from those of 
Hyphydrus. External marks of the sexes are scarcely to be found, and the colour 
of the upper surface is without the variegation existing in Hyphydrus. 
The Pachydri are peculiar to tropical America and the West Indian Islands. 
I. 21.—Genus DESMOPACH RIA. (Vide p. 340). 
About twelve species form this aggregate ; they are minute or small insects, of 
short, broad form, convex beneath. Front of head margined, the clypeus overhang- 
ing the labrum, but when viewed from beneath the latter is seen to be distinctly 
exserted. Prosternum extremely minute, forming in fact merely a slender frame 
for the coxze : prosternal process minute, rhomboidal, acuminate behind. Middle 
coxa distinctly separated. Metasternum very long in the middle, with its curved 
side wing excessively slender, so as to be a mere line, the suture between it and the 
coxa is however excessively obliterated so that this character is not easily 
appreciated. Hind coxee enormous, their processes quite adpressed to the ventral 
segment, moderately broad, and leaving quite exposed the coxal cavities; hind 
border soldered to first ventral segment. Hind legs moderately stout ; their rather 
stout short tibiz of even thickness throughout. Front and middle tarsi small, four- 
jointed, third joint but little lobed. Elytra on inner face at extremity with a single 
prominent ligula. 
TRANS. ROY. DUB. SOC., N.S., VOL. II. 5 R 
