842 On Aquatic Carnivorous Coleoptera or Dijtiscidce. 



acuminate spine ; the front coxaj are very ap[)roximate, and the presternum in 

 front continues the plane of direction of the prosternal process ; the mesosternum 

 is placed almost completely at right angles to the metasternum ; its epimeron is 

 laro-e, and very broad at the metathoracic-episternal angle ; its fork is small and 

 inconspicuous. The metasternum is elongate in the middle, but excessively 

 reduced at the side ; it approaches very near to the middle coxa, and is afterwards 

 deflexed outside the anterior border of the hind coxa, forming a slender very curved 

 band ; the iuter-coxal process is very narrow owing to the great approximation of 

 the middle coxae, and is curved up in front to form a minute but perfect articulation 

 with the mesosternal fork, it is marked by a very fine groove (of variable length) 

 for the i-eception of the extremity of the prosternal process. The hind coxa? are 

 of enormous size, and extend greatly forwards so as to possess an extremely arched 

 anterior border ; the coxal lines are of peculiar form, they are very distinct, and 

 commence in front at the apex of the metasternum being tliere much approximate 

 to one another, they extend backwards, diverging from one another till each reaches 

 the outer hind angle of the process ; the two processes are completely soldered 

 quite to the hind margin, which thus presents a truncate edge ; the coxal border is 

 very distinct and definite, it is broad in front but terminates in a point behind : 

 the coxal cavities are completely contiguous and very much concealed and protected 

 by the processes ; no coxal notch can be detected, unless a slight sinuation of the 

 hind margin of the process be considered to represent it. The anterior and middle 

 legs are feeble, their tarsi conspicuously 5-jointed and without any trace of lobing 

 of the third joint. The swimming legs are highly developed and powerful, their 

 femora are broad and are laminate at their hinder outer angle, which is nearly a rect- 

 angle and well marked ; the tibiae are short and stout, much shorter than the femora, 

 and terminated by two sublinear highly developed spurs, which are not acuminate, 

 but are minutely emarginate at the extremity; the tarsi are of peculiar and 

 characteristic form ; when their outer or upper face is observed it is seen that the 

 hind margin of each joint deviates greatly from a straight line, the outer portion of 

 the joint being produced backwards, so as to form a kind of lobe projecting over the 

 following joint ; the basal joint is about as long as the two following ones together, 

 the terminal one is broad, and bears a single short stout claw. 



The abdominal stigmata are all small and circular. 



The male tarsi are but little developed, sometimes it is not easy to distinguish 

 any difference between their structure and those of the female ; they show very little 

 dilatation and when they are thickened the incrassation is quite as great in the 

 vertical as it is in the horizontal direction ; no sexual sculpture has been detected 

 in the female. 



An interesting feature in certain species of this genus is the existence of a stridu- 

 iating structure consisting of a curved series of grooves on the hind coxa ; these 

 are played on by a single, very obscure elevation situated on the anterior edge of 



