On Aquatic Carnivorous Coleoptera or Dytiscide. 901 
the fourth and following ventral segments are very narrow; the metasternal 
groove is narrow and indistinct ; the terminal joint of the hind tarsus is not longer 
than the preceding one. The prosternal process is short and comparatively broad, 
and distinctly compressed: the metasternal groove is indistinct, its lateral margins 
being ill defined owing to the great approximation of the middle coxee: the side 
wings of the metasternum are large, and their anterior border is very little arched : 
the coxal processes have much extension in the transverse, and but little in the 
longitudinal direction ; the swimming legs are rather long and slender ; with the 
lamina at their extremity obscure and its angle almost rounded, their tarsi have 
the hind margins of their joints strongly lobed externally, and their claws are very 
unequal in length ; the prothorax has no lateral margin; the second ventral seg- 
ment bears a file which in some species is highly developed, but in others is fine 
and inconspicuous. The penultimate abdominal stigma is large and transverse. 
The claws of the male front tarsi remain short, and never show the tendency to 
elongation so frequent in Rhantus. 
The greater development of the penultimate abdominal stigma, and the contracted, 
indefinite groove on the metasternum seem to distinguish this aggregate in a posi- 
tive manner from the preceding one, Rhantus. 
The male tarsi are variable, and in accordance with their structure the species 
may be arranged in four groups as follows :— 
Group 1. (Four species, Nos. 960 to 963).—Male tarsi much dilated, not 
compressed, densely clothed beneath with glandular hairs, but without any 
palettes; ventral file variable ; transverse sculpture of elytra very distinct. 
Group 2. (Three species, Nos. 964 to 966).—Male tarsi much dilated, not com- 
pressed, clothed beneath with three series of well developed palettes, and a 
basal band of glandular pubescence. Transverse sculpture very distinct. 
Group 8. (Seven species, Nos. 967 to 973).—Male tarsi much dilated, not 
compressed, clothed beneath with four series of well developed palettes, and 
a basal band of glandular pubescence. Transverse sculpture of elytra very 
distinct. 
Group 4. (Four species, Nos. 973 to 977).—Male tarsi but little dilated, much 
compressed, with palettes beneath, the heel withouc glandular pubescence ; 
transverse sculpture of elytra very slight or entirely absent. 
The species of the first three groups are nearly all very rare in collections and 
confined to the northern regions of the New and Old Worlds, some extending into 
Arctic regions ; one species of the third group is however excessively abundant 
throughout all Europe. The species of the fourth group are confined to the 
Mediterranean region of the Old World. 
5 Z 2 
