Mountain beavers are the only extant member of the family Aplodontidae, and are considered the most primitive living rodents. Mountain beavers are stout, compact and cylindrical and have a broad, massive, laterally compressed skull. They average about one foot in length and two to four pounds in weight. Mountain beavers have small eyes, rounded ears, and a distinctive cylindrical stump of a tail. Each forepaw has an opposable thumb and all digits have long curved claws for digging. Mountain beavers are not closely related to true beavers (Castor spp.), which are semi-aquatic. The Point Arena mountain beaver is one of seven subspecies of mountain beaver. Certain cranial and other characteristics separate the Point Arena mountain beaver from other subspecies of mountain beavers. The most obvious of these characteristics is the unique blackish coloration.