A deputy from the San Mateo County Sheriff's Office called in the Coroner's Office, and a deputy coroner estimated that the skull was "very old and of Native American descent," according to a Sheriff's Office report.
The deputy coroner collected and took possession of the skull and other remains found with it, the report said.
In July, a worker found part of what the Coroner's Office considered an old Native American skull while digging a trench near a swimming pool in the 100 block of Aliso Way in the unincorporated community of Ladera, a few miles to the west of West Menlo Park.
In that case, investigators found no other remains, just the partial skull.
As George Mader, a consultant to Portola Valley's planning department, said in July, Native American remains are not uncommon near creeks. In the Ladera case, the Los Trancos Creek runs nearby; in West Menlo Park, San Francisquito Creek is within a mile of the most recent discovery.