Hydrophilus, as heretofore recognized in our fauna, contains two sets of species, the one series large, the other relatively small. They also differ in the form of the maxillary palpi and should properly be considered distinct genera defined as follows:—

Hydrophilus.—Terminal joint of maxillary palpi much shorter than the penultimate, the second joint long, arcuate. Claws toothed.

Tropisternus.—Terminal joint as long or even longer than the penultimate, the second joint moderately long and straight. Claws not toothed.

The large species belong to the first series, and but two have appeared in our lists.