On Aquatic Carnivorous Coleoptera or Dytiscide. 899 
horni this piece is only about 23 times as long as broad. In many other respects 
however the genus undoubtedly comes near Meladema. The prothorax is without 
side margin; the prosternal process is rather broad, distinctly compressed ; the 
metasternal groove is imperfectly developed; the wings of the metasternum are 
not large, (in Scutopterus horni they are decidedly small); the coxal lobes have a 
considerable extension in the transverse, but little in the longitudinal direction. 
The swimming legs are rather slender especially the tarsi, and the lamina of the 
femora is rounded at its outer angle; the terminal joint of their tarsi is elongate, 
longer than the preceding one, and their lower claw is very long, twice as long as 
the other. 
The species of this interesting and isolated genus are peculiar to the northern 
parts of North America, and are still rare in collections. 
I. 62.—Genus RHANTUS. (Vide p. 607.) 
This is an extensive aggregate, with forty species: the size of the individuals is 
moderate {from 8 to 15 m.m. of length). The side pieces of the fourth and following 
ventral segments are very narrow; the metasternal groove is distinct and well- 
defined, and the terminal joint of the hind tarsus is not longer, (or is but little longer) 
than the preceding one, 
The numerous species are distinguished constantly from their allies by the above 
characters, as well as by two others, which are not quite so constant, these are first 
that the prothorax has a distinct lateral margin, Nos. 957, 958, and 959 being 
however exceptions in this respect; and second, that the swimming legs are rather 
well developed, the femora having their undersurface forming a developed lamina 
towards the extremity. The colouration of the upper surface usually is yellow, 
with black specks on the wing cases, but in some species it is entirely black, and in 
other cases nearly so ; where the wing cases are nearly but not quite entirely black 
it is generally the margins that are pale, and the black colour seems to have been 
produced by a coalescence of the black specks. The prosternal process is moderate 
in length, never elongate, nor very short, and neither slender nor broad, always 
distinctly compressed, The metasternal groove though not elongate is perfectly 
well developed, the middle coxze never being so much approximated as to interfere 
with its development, and it has therefore always perfectly distinct sides ; the coxal 
lobes are broad and the division between them extends far forwards (except in 
Colymbetes capensis (No. 957) and Rhantus goudoti (No. 958), which depart from 
the other species in this respect ; the supra articular border is narrow ; the coxal 
lines are much separated in front ; the side wings of the metasternum are variable in 
size, but are never very small. The swimming legs are well developed in some species 
such as Rhantus goudoti (No. 958), and Dytiscus calidus (No. 942), but are more 
slender in others, especially in Colymbetes pacificus and other species placed near the 
rp 
TRANS. ROY. DUB. SOC., N.S., VOL. I. iy, 
