On Aquatic Carnivorous Colcoptera or DytiscidcB. 921 



an oval, attenuate behind, but vaiying from extremely short to elongate ; the upper 

 surface may be punctate, but is usually excessively polished, the wing-cases are 

 variegate, or not so ; the prominent platform of the breast is nearly always very 

 coarsely punctured. The antennae show no remarkable developments ; the front 

 tibise have the outer margin lounded and densely set with short regular cilias, the 

 outer apical angle has completely disappeared ; moreover tlie tibia is narrowed to- 

 wards the apex, and on its posterior face is a large cavity in which is inserted a thick 

 curved spur ; owing to the attenuation of the tibia, this spur has often the appearance 

 of being an actual prolongation of the tibia. The anterior coxae are always a good 

 deal separated from one another, the anterior transverse band of the prosternum is 

 in front of the coxse moderately long, the prosternal process proceeds backwards 

 from this band from an almost insensible point of departure, and behind the coxsb- 

 becomes greatly broader (Hydrocanthus) or gently broader (Canthydrus) and 

 its posterior margin is truncate, or only very obscurely a little longer in the 

 middle. 



The ooxal processes are greatly prolonged backwards, and their terminal angle is 

 rendered acute by a group of rigid cilias. The swimming legs are either moderately 

 or greatly developed ; their femora have at the extremity of their hind margin a 

 very highly developed group of cilise ; the face of the femur is either traversed by 

 a series of punctures parallel to, but at a distance from, the hind margin (Hydro- 

 canthus), or placed so very close to the margin as to give the appearance of the 

 hind margin itself being ciliate (Canthydrus). 



The Hydrocanthini are distributed over the warmer parts of the world ; in the 

 European region they do not extend north of the Mediterranean, and in the Ameri- 

 can are found at any rate as far north as Massachusetts. 



So far as regards the distinction of the species, the Hydrocanthini are amongst 

 the most difficult of insects to deal with ; the species however, notwithstanding 

 their extreme similarity, will be found, I have no doubt, to be really distinct, 

 and probably will prove to be much more numerous than is at present 

 anticipated. 



II. 4.— Group Vatellini. {Vide ^^. 282.) 

 This group comprises only nine species, arranged in three genera, two of which 

 are autogenera. They are insects of rather small size, (from 3 to 8 m.m. in 

 length) ; of Hydroporoid appearance, but with the outline of the body much more 

 interupted at the junction of the thorax and elytra than is usual in the Dytiscidse ; 

 the front and middle legs are elongate, and the front tarsi are peculiarly elongate 

 and narrow in some of the species. The head is very short in front of the large 

 and prominent eyes ; the prosternal process is very abruptly bent, and does not 

 connect with the metasternum, but its point terminates in front of the middle coxae, 

 between which it is concealed ; the middle coxae are exposed, and prominent, and 



