. KEY TO THE GENERA OE ADULT DYTJS- 

 CIDAE— DIVING BEETLES. 



1. Third joint of the fore and middle tarsi deeply bi- 



lobed, the fourth joint rudimentary or wantin^^. 2 

 Third and fourth joints not greatly different from 

 the others 8 



2. Scutellum visible (S. U. S.) • • (2) Celina 



Scutellum invisible 3 



3. Hind coxal processes each divided by a deep pos- 



terior notch, the inner ramus oppressed against 

 the first abdominal segment (6) . . . . HydrovaUis 

 Hind coxal processes not so formed 4 



4. Hind margin of outer lamina of hind coxa grown 



solidly coherent by its margin with the first ven- 

 tral segment of the abdomen 5 



Hind margin of hind coxa overlapping but not 

 coherent with the first ventral segment of 

 abdomen 6 



5 . Prosternal process broad and short with obtuse hind 



margin; middle coxae conspicuously separate 

 (16) Bidcssiis 



Hind end of prosternal process rhomboidal; middle 



coxae more approximate (4) Desmopachna 



b. A thin flat angulate process springing from the longi- 

 tudinal ridge that extends along the under side of 

 the wing covers near their side margins (visible 

 only if the wing covers are lifted and viewed from 

 beneath) (18) Coelamhns 



No such free process beneath the wing covers 7 



7. The mesosternal fork connected with the intercoxal 

 process of the metastemum (72) . . . .Hydroporus. 



The mesosternal fork not connected with the inter- 

 coxal process of the metasternum(4) . .Dcronecics. 



