On Aquatic Carnivorous Coleoptera or Dytiscidce. 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 coxse : 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 dii'ection ; 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 latei'al margin ; the second ventral seg- 

 ment bears a file which in some sjjecies 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. 9G0 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. 9G4 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 S. (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 without 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. 



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