40 COLEOPTERA OF NORTH AMERICA. 



Four tribes are represented in our fauna, which may be thus 

 distinguished: — 

 Anterior and middle tarsi with the third joint bilobed, the fourth scarcely 



visible. I. Hydkoporini. 

 Tarsi with five distinct joints ; 



Presternum dilated behind, truncate. II. Noterini. 



Presternum compressed ; 



Anterior tarsi of % dilated, joints oblong. III. Colymbetini. 



Anterior tarsi of % forming a large disk. IV. Dytiscini. 



Tribe I— HYDROPORIjVI. 



This group contains only small species, having the usual form 

 of this family, but rarely very broad, and sometimes nearly globose. 

 But two genera are found with us: Hydroporus, of varied form, 

 having no visible scutellum ; Celina, with elongate body, obtuse 

 before, pointed behind, with a distinct scutellum. 



More than eighty species of the former genus are known to me,* 

 and species are found in every part of the country. 



• Tribe II.— KOTERIXI. 



The form of the prosternum in Noterus and the three genera 

 below mentioned is so remarkably different from that seen in the 

 rest of the members of this family, that they seem to be naturally 

 placed in a special tribe; the prosternum is elevated, very much 

 dilated behind the anterior coxa?, and truncate ; it reaches the 

 metasternum, which is also widely truncate in front. 



The form of body in these genera is likewise peculiar. In Ool- 

 pius Lee. it is almost globose, as in certain Hydroporus; in the 

 others it is oval, very convex, acutely pointed behind; the labial, 

 palpi are dilated in our genera, sometimes very much so ; the an- 

 tennas have the middle joints more or less dilated. The scutellum 

 is not visible. No species has yet occurred in the Pacific district. 



Our three genera may be thus distinguished: — 



Presternum deeply concave ; body globose ; palpi emarginate at tip. 



Colpips. 

 Prosternum flat ; 



Last joint of maxillary palpi emarginate. Sr/pnis. 



Last joint of maxillary palpi truncate. Hydeocanthus. 



Colpius contains but a single species, found in Louisiana. 



* For a synopsis of them, vide Proceedings of the Academy of Natural 

 Sciences of Philadelphia, vii. 290. 



