88 Bulletin American Museum of Natural History [Vol. XLII 



6. Larger in size, more than 25 mm. long; one supraorbital stylus; only 16 well- 



developed pronotal styli but several other inconspicuous ones; cerci 1 nearly 



3.5 mm. long, superficially annulate, and not bifid (?) Hydrous. 



Smaller in size, less than 12 mm. long; no supraorbital styli; 22 well-developed 

 pronotal styli; cerci about 1.45 mm. long, superficially annulate, slightly 

 bifid and acute Tropisternus. 



7. Larger in size, more than 13 mm. long; pronotal styli numerous, 32 in number; 



eighth abdominal tergite with two, small, rounded tubercles, each bearing a 



short terminal stylus; cerci slightly bifid and acuminate Hydrophilus. 



Smaller in size, less than 8 mm. long; pronotal styli fewer, 24 in number; 

 eighth abdominal tergite without the tubercles mentioned above but with a 

 pair of styli ; cerci not bifid, acuminate or thread-like distally 8 . 



8. Smaller in size, less than 3 mm.; styli are short with long terminal setae; the 



styli at the anterior margin of the pronotum 10 in number, four in the middle 



and three towards each side Paracymus. 



Larger in size, more than 3 mm.; styli not noticeably short nor the terminal 

 seta long, about equal to each other; the styli at the anterior margin of the 

 pronotum 6 in number, two in the middle and two towards each side; the 

 two end styli on each side, which were present in the above, have moved to 

 a distinct lateral position Cymbiodyta; Philydrus; Helochares. 2 



9. Abdominal pleurites often confused with the tergites; cerci long, slender, 



acuminate, multiannulate Cercyon. 



Abdominal pleurites distinct; cerci short, conical, two-segmented . . Sphwridium. 



V. LITERATURE 

 Baker, W. F. 1894. Hydrobius fuscipes, metamorphoses, larval anatomy, etc. 



Naturalist, pp. 327-333. 

 Balfour-Browne, Frank. 1908. British species, claws, etc., of Philhydrus. 



Ent. Rec, XX, pp. 25-29, PI. iv. 

 1910. On the life-history of Hydrobius fuscipes L. Trans. R. Soc. Edinburgh, 



XLVII, pp. 317-340, PI. in. 

 1913. The Life-history of a water-beetle. Nature, XCII, pp. 20-24. London. 

 Barber, H. S. 1906. The Spread of Splmridium scarabaioides . Proc. Ent. Soc. 



Washington, VII, pp. 127-128. 

 Berlese, Antonio. 1909. Hydrobius fuscipes; Hydrophilus piceus; Berosus 



spinosus. Gli Insetti, I, pp. 55, 832, 875. 

 Beutenmuller, William. 1891. Bibliographical catalogue of the described trans- 

 formations of N. A. Coleoptera. Journ. N. Y. Micros. Soc, VII, pp. 5-6. 

 Blatchley, W. S. 1910. Family Hydrophilidse, the water scavenger beetles. 



Coleoptera of Indiana, pp. 247-271. 



J The cerci are broken off in the specimen at hand. They are probably not bifid, as Matheson, 

 1914, does not mention it in H. triangularis. 



2 There appear to be nc distinct characters for the separation of these three genera. The arrange- 

 ment of the pronotal seta? differ slightly but cannot be incorporated into the key. Examination of the 

 metathoracic tarsus shows, however, the four-segmented condition in Cymbiodyta and the five-seg- 

 mented in Philydrus and Helochares. 



