238 GEO. H. HORN. 



The tribe Hydroliiini has been restricted by Bedel to Laecobius, 

 Philydrus and Hydrobius, together with any subdivisions of the last 

 two. Helopeltis may be removed by the form of the head (PI. iv, 

 rig. 21) and concealed labrum. Dr. Sharp observes that the anterior 

 coxal cavities are closed behind, while in the other genera they are 

 open. 



The present study is practically narrowed to the species formerly 

 considered by me as belonging to Pliilydrus and Hydrobius, and as 

 any genera which follow are dismemberments from these, the discus- 

 ,sion is best begun by giving the characters separating them : 



Terminal joint of maxillary palpus rarely as long, usually shorter than the pre- 

 ceding joint PHILYDRUS. 



Terminal joint of ma.\-illary palpus always longer than the preceding joint. 



HYDROBIUS. 



It will be observed in all the species of Hydrobius as above de- 

 rined, that the terminal joint of the maxillary palpus always bends 

 inward toward the other joints and in the same plane as is usual 

 with palpi generally, but in Philydrus the terminal joint is so articu- 

 lated that it bends downward as well, and is less inclined to bend 

 toward the mouth. 



The aggregate above separated as Philydrus has been divided into 

 six jiarts, defined by the following chai'acters, which, for convenience, 

 are reduced to a tabular form. 



Tarsi .o-jointed on all the feet 2. 



Tarsi 4-jointed on the middle and hind feet 4. 



2. — Second joint of maxillary palpi* curved with the convexity to the front; 



mesosternuiu laminate PHILYDRUS. 



Second joint curved witii the convexity posterior 3. 



3. — Mentum with anterior horderentire ; mesosternum with median longitudinal 



lamina; elytra without sutural stria SINDOLUS. 



Mentum with a slight notcht at middle ; mesosternum not carinate: elytra 



striate or not HELOCHARES. 



Mentum regularly impressed at middle, the middle of the anterior l)order 

 deeply semicircularly emarginate ; mesosternum prominent in a longitu- 

 dinal manner along the middle; sutural stria distinct.-CHASMOGENUS. 

 4.— Mesosternum with a slight transverse elevation, a little in front of the coxie ; 

 prothorax without basal marginal Hue; elytra with sutural stria. 



HYDROCOMBUS. 



Mesosternum longitudinally carinate ; prothorax without basal niargiiial line ; 



elytra with .sutural stria CYMBIODYTA. 



* Called for convenience in the subsequent pages "pseudo-basal joint." 

 t " But the structure of the neighboring parts renders this character very 

 difficult of verification (Sharp, Biol, i, 2, p. 74)." 



