922 On Aquatic Carnivorous Coleoptera or Dytiscide. 
quite contiguous, there being no inter-coxal prolongation of the front of the meta- 
sternum. The mesosternum is more or less exposed between the metasternum and 
prothorax. The hind coxe are large, their front border has a considerable extension 
in the anterior direction, and its most forward point lies very near the outside of the 
body. The hind coxal cavities are not contiguous; and the swimming legs are slender. 
The front and middle tarsi are peculiar, the three basal joints are elongate, more 
or less compressed laterally, the third joint is scarcely bilobed, but has an emargina- 
tion at the apex, in which is inserted the fourth joint, the terminal joint is therefore 
exserted, it is not very long, but is very slender, and is terminated by two very 
small claws; at the base of this (truly the fifth joint), the true fourth joint may be 
detected, although it is very small and minute ; the joints of this tarsus are very 
loosely articulated, so that the foot has a very fragile appearance ; there is in some 
species a good deal of sexual disparity in the structure of the tarsi, and in the 
males of some species of Macrovatellus, the peculiar form of the tarsus is little 
apparent, and departs but little from what exists in the Hydroporini. 
The three genera when examined are found to be very distinct inter se, Derova- 
tellus is a small insect, Hydroporoid in form, and has the mesosternum but little 
exposed, the head is very short in front, and has no anterior band inflexed over 
the labrum ; its head is in fact Hydroporoid, while in Vatellus this part is more 
Hyphydroid, there being an inflexed anterior edge placed at right angles to the 
plane of the upper surface; this genus, Vatellus, is highly remarkable on account 
of the deep sutures of the ventral segments. 
In Macrovatellus the individuals are of comparatively large size, and the front 
tarsi are not so slender as in the other two aggregates, while on the other hand 
in it the exposure of the mesosternum reaches its maximum; in respect of its head 
and the posterior coxal cavities, it may be considered as intermediate between 
Vatellus and Derovatellus. 
The Vatellini form a very natural and interesting aggregate ; although on account 
of the structure of the intermediate cuxal cavities the group belongs to the Dytisci 
fracmentati, yet it has no other relationship with the components of that series ; 
it may be considered to be the analogue in the Dytisci fragmentati, of the 
Hydroporides in the Dytisci complicati. The Vatellini show in fact several points 
of approximation to the Hydroporides, but these approximations are to different 
members of that tribe, not to any one form thereof, and the approximation is never 
in a number of characters, but only in a single point. Thus the peculiar prosternal 
process not connecting with the metasternum, reappears in Bidessini (Tyndalhydrus) 
and in Hyphydrini (Andex). The peculiar tarsi are somewhat similar to those of 
Hyphydrus very much elongated, and this point is also approximated to some 
extent by Necterosoma of the Hydroporini. The widely different and isolated 
Sternopriscus approximates the Vatellini by the exposure of the mesosternum. 
One of the most peculiar of the European species of Hydroporus (Dytiscus dorsalis 
