62 COLEOPTERA OP NORTH AMERICA. 



front; gular suture distinct; ligula large, quadrate, cor- 

 neous. 



Maxillas with the outer lobe biarticulate, the inner curved, 

 acute at the apex, ciliate internally. 



Eyes rounded, never emarginate. 



Antennce inserted under the front, behind the base of the 

 mandibles, glabrous, polished, usually filiform, 11-jointed. 



Prothorax with the epimera and episterna distinct; pro- 

 sternum compressed, produced behind and fitting into a cleft 

 or emargination of the metasternum ; anterior coxte pro- 

 tected behind by the mesosternum, subconical. 



Metasternum short, pointed behind, but very closely con- 

 nate with the posterior cox£e, without ante- coxa! piece. 



Posterior coxae very large, usually oblique, contiguous at 

 the inner margin, reaching the side of the body, entirely 

 cutting off the ventral segments from the metathorax ; in- 

 ternally with a small dilatation for the insertion of the legs. 



Abdomen with six ventral segments, tlie three anterior 

 ones connate, the sixth rounded at tip, usually permitting 

 the seventh internal, but corneous one, to be slightly visible. 



Legs ciliate with long hairs, posterior usually compressed, 

 elongated, Ibrmcd for swimming; tarsi 5-jointed, the fourth 

 joint of the anterior and middle tarsi sometimes obsolete. 



In tills family are contained aquatic carnivorous insects, having, 

 as will Ije seen by the above characters, a close relationship to 

 CarabidfE, and in fact only differing by the form of the meta- 

 sternum, the posterior coxaj, and the natatorial legs. The par- 

 ticular portion of the Carabld* which approaches most nearly 

 these insects is found in some tribes of the Carabinae. 



The Dytiscidse, following the system of Dr. D. Sharp, who has 

 in press a very exhaustive memoir on the species of this family, 

 may be divided into two series, by a character somewhat similar 

 to that used so effectively in the primary division of the Carabidaj. 



Metathoracic episteruum not reaching the middle coxal cavity. 



D. FRAGME\TATI. 



Metathoracic episternuni reaching the middle coxal cavity. 



D. COMPLICATI. 



Series I. — DytiscidjE fragmentati. 



The genera in our fauna indicate but two tribes; in both of 

 which the scute! is invisible. 



