64 COLEOPTERA OF NORTH A3IERICA. 



ing into the articulation. They are to be regarded as the highest 

 Uytiscide type, in which not only the maximum size and force is 

 exliibited, but also the most perfect development of the oar-like 

 hind legs. The following tribes occur in our fauna. We have 

 somewhat changed the tabular arrangement given by Dr. Sharp 

 of the tribes of this series, so far as they are represented with us. 



Prostemum not dcflexpd hetAveen tlu^ front coxae; tarsi distinctly 

 r)-jointed. 2. 



Presternum deflexed ; front cand middl«_> tarsi 4-jointed, oi- apparently so. 



IIydroporini. 



2. Front tarsi of % with dilated joints forming a round disk. 3. 

 Front tarsi of 'J, with dilated joints oblong. Colymbetini. 



3. Posterior pairs of spiracles large, transverse. Dytiscim. 

 Posterior pairs of spiracles small. Cybistrini. 



Tribe I.— HYDROPORIXI. 



The species are of small size, and very numerous; they are 

 easily known by the 3d joint of the front and middle tarsi deeply 

 lobed, concealing the 4th joint, M'hich, however, is most frequently 

 wanting; the 5th joint is slender, with claws which sometimes 

 vary in form according to sex. The sculpture is also in man}- 

 instances quite different in the sexes, so that some care must be 

 taken in separating the species. 



The genera in our fauna are as follows: the categories 1-4 

 represent separate groups, for the definition of which, vide the 

 great memoir of Dr. Sharp, above mentioned. 



Hind coxal cavities not excised. 2. 



Hind coxal cavities distant, excised. Hydrovatus. 



2. First ventral segment connate with tlie hind coxse, which are not con- 



tiguous. 3. 



First ventral segment free. 4. 



3. Prosternal process rhomboidal, acute at tij). Desmopachria. 

 Prosternal process oblong. Bidessus. 



4. Scutel not visible. 5. 

 Scutel distinct. Celina. 



5. Elytral ligula distinct, abrupt ; metasternum not attaining tln' nieso- 



sternum. Coelambus. 



Elytral ligula wanting; metasternum not attaining the mesosternuni. 



Deronectes. 

 Elytral ligula wanting; metasternum attaining the mesosternuni. 



Hydroporus. 



