A resident of Lynn Court found the skull around 4 p.m. while clearing vegetation in his yard, according to police.
The skull was turned over to the Marin County coroner's office for testing. On Tuesday, the coroner's office reportedly determined the skull did not belong to a historical Native American.
Forensic anthropologists will conduct further extensive tests in order to determine the gender, race and age of the deceased, police said. The tests may take up to a month to complete.
A search of the hillside where the skull was found is expected to continue today, according to police. Members of the Marin County Search and Rescue team, San Rafael police and crime scene investigators will participate in the search.