NORTH AMERICAN COLEOPTERA. 115 



Elytra entire PTILIINA. 



Pronottiin widest at the base. 



Pygidiini invisible Nossidiiim. 



Pygidiiun visible beyond the elytra. 



Posterior angles of pronotum elongate Tliroscidiuiii. 



Posterior angles of pronotum not elongate KaiKiNolla. 



Pronotum widest before the base. 



Base of pronotum fitted to tlie shoulders of the elytra. 

 Metasternuiii extending to the sides of the body. 



Pygidiuni visible beyond the elytra 1*1 ilium. 



Pygidium invisible Pteiiidiiiiii. 



Metasternuin not reaching the sides of the body. 



Pronotum not constricted at the base MotschulHkiiiin. 



Pronotum constricted near the base. 



Pronotum very deeply sulcate 9Iillidiiiiii. 



Pronotum not sulcate Micridiiiin. 



Base of pronotum suj)erincumbent on the shoulders of tlie elytra. 



Actidium. 



TRICHOPTERYGID^. 



HYDROSCAPHINA. 

 HYDKOSCAPHA. 



LeConte Trans. Amer. Ent. Soc. 1874, p. 45. 



Body conic, very elongate. 



Antenna very short, 8- or probably 10-jointed, apical joint alone in- 

 crassate, apparently composed of three amalgamated articulations. 



Head sessile, deflexed. 



Pronoti'M short, much dilated at the base. 



Elytra truncate, very much attenuated tovpards the apex. 



Abdomen very long, in form like Tdchi/ponts, retractile, and therefore 

 in dried specimens scarcely visible beyond the elytra, 



Me.sosterni:m broadly carinated, carina anteriorly acuminate. 



Metasternum large, transverse, not extending to the sides of the body. 



Venter 8 segments, apical segment deeply emarginate, containing a re- 

 tractile laminated apparatus for swimming. 



Legs robust, posterior tarsi very long. 



Posterior cox^ widely separated, broadly laminate. 



H. natans LeConte, Trans. Amer. Ent. Soc. 1874, p. 45. — L. c, abdomen re- 

 tracted, 1 mm.; abdomen extended, 1.75 min. — Conic convex, very shining, pi- 

 ceous very sparingly clothed with short griseous hair ; head large, rounded in front, 

 eyes large, thorax short_, transverse, rather longer and much broader than the head, 

 widest at the base, marked with remote punctures, sides slightly rounded and nar- 

 rowly margined, hinder angles almost right angles; elytra very convex, much at- 

 tenuated toward.s the apex, rather wider and much longer than the head and 



