This skull was a secondary find during last Saturday's field trip for Cepaea nemoralis. We were in a wooded lot next to a river, making it likely that it was a beaver skull. At the same time, not too far from the spot there were large tunnels dug into the soil at the edge of the forest alongside an open field, making it likely that it was a groundhog (woodchuck) skull.
A comparison of the skulls of the 2 species, let me identify it as a groundhog skull without any doubt. The groundhog (Marmota monax) is on the left, the beaver (Castor canadensis) is on the right. The most obvious difference is the "postorbital processes" of the groundhog skull that are missing in the beaver skull and which are present in mine.
Pictures from Mammalian Species.
The skull of a distant contender, the rabbit, eastern cottontail (Sylvilagus floridanus), is quite different from either species.