940 FRESH-WATER BIOLOGY 



39 (36) Mesosternum with a longitudinal carina; all tarsi with five segments, 

 the first one small Philhydrus. 



4 (33) Terminal segment of maxillary palpi distinctly longer than the 

 penultimate. 41 



41 (42) Large species 6.5-8.5 mm.; the elytra distinctly striate or striato- 



punctate Hydrobius. 



42 (41) Small species 1.5-3.5 mm.; the elytra not striate but with confused 



punctuation . Creniphilus. 



43 (24) Metasternum prolonged into an acute spine; tarsi compressed so as 



to be oarlike. The color is always pitchy black, occasion- 

 ally with yellow margins. The size varies from 9 to 35 

 mm . '.. " 44 



44 (45) Metasternum produced into a short spine never projecting as far 



as the posterior margin of the first ventral segment. Pro- 

 sternum acutely carinate but not grooved for the reception 

 of the mesosternal carina Hydrocharis. 



45 (44) Metatarsal spine very long and acute, extending always beyond the 



posterior margin of the first ventral segment. Prosternum 

 with a keel-shaped process which is deeply grooved for the 

 reception of mesosternal carina, thus locking pro- and meso- 

 thorax together 46 



46 (47) Length about 10 mm. Terminal segment of maxillary palpi as long 



as or longer than the preceding; the antepenultimate seg- 

 ment straight; claws simple Tropisternus. 



47 (46) Length about 35 mm. Terminal segment of maxillary palpi much 



shorter than the preceding; the antepenultimate segment 

 arcuate; claws toothed Hydrophilus. 



48 (15) Antennae longer than the palpi (equaling them in a few riffle beetles). 



49 



49 (90) Hind coxae broadly flattened out and solidly fused with the meta- 



sternum 50 



50 (51) Metasternum divided by a transverse suture which separates a short 



sclerite before the base of the hind coxae. 



Family AMPHIZOIDAE. 



A single genus from western mountain streams. Amphizoa. 

 51(50) Metasternum not thus divided Family DYTISCIDAE . . 52 



52 (61) Scutellum invisible 53 



53 (54) Third and fourth segments of the fore and middle tarsi not greatly 



different from the others; prosternal process acute posteriorly. 



Laccophilus. 



54 (53) Third segment of the fore and middle tarsi deeply bilobed, the fourth 



segment rudimentary or wanting 55 



55 (56) Base of thorax united to the elytra by a short impressed line on each 



side, continued without interruption across the border of 

 each; hind margin of the posterior coxae grown solidly 

 coherent with the first ventral segment of the abdomen, 

 which is considerably enlarged; form elongate; very small. 



Bidessus. 



56 (55) Base of thorax and elytra without a continuous impressed line. . 57 



