106 Mr. D. Sharp's revision of the species 



19. Tropisternus oralis, Cast. 



Cast., Hist. Nat., ii., p. 54 ; Sharp, Biol. Cent. Am., 

 i., pt. 2, pi. ii., f. 5. 



From Mexico to Bahia. Oaxaca ; Guatemala ; Co- 

 lombia (Coll. Reiche) ; Bahia (Coll. Castlenau). 



The individuals of this species are of smaller size 

 than others of the genus, except the smallest individuals 

 of T. nitens ; the form is peculiarly short and broad, the 

 greatest width being near the extremity behind, and the 

 punctuation of the upper surface is more distinct and 

 less dense than usual ; these characters, in combination 

 with the unusual development of the 6th joint of the 

 antenna, permit the easy identification of the species ; it 

 appears to be rare in collections. 



Section B. 



Posterior tibiae with an elongate series of swimming- 

 hairs placed in a groove extending along the outer mar- 

 gin of their upper-inner face, from the knee to near the 

 extremity. 



This section comprises groups 7 to 10. The species in 

 this section are more closely allied inter se than are many 

 of the species of the A section, and I have separated 

 them into groups merely to facilitate the determination 

 of species. 



Group 7. Upper surface without yellow cincture, the mar- 

 gins being nearly or quite concolorous ; epipleural 

 margin distinctly punctate from the humeral angle to 

 near the extremity. (Species 20 to 27). 



In this group I have placed eight species ; the first 

 two of these, Tropisternus concolor and Hydrophilus 

 ellipticus, have the lateral systematic punctures on the 

 thorax reduced to one or two on each side, while in all 

 the other species they form a short, sometimes irregular, 

 line of five or six punctures. In all the species the 

 sternal spine is of moderate length, or is rather elon- 

 gate ; it is never short or flattened beneath ; the pube- 

 scent area at the base of the hind femur is either small 

 or very small. The armature of the last ventral seg- 

 ment is very variable in this group of species. 



