Given the following context:  San Francisco—officially known as the City and County of San Francisco—is a consolidated city-county, a status it has held since the 1856 secession of what is now San Mateo County. It is the only such consolidation in California. The mayor is also the county executive, and the county Board of Supervisors acts as the city council. The government of San Francisco is a charter city and is constituted of two co-equal branches. The executive branch is headed by the mayor and includes other citywide elected and appointed officials as well as the civil service. The 11-member Board of Supervisors, the legislative branch, is headed by a president and is responsible for passing laws and budgets, though San Franciscans also make use of direct ballot initiatives to pass legislation. The members of the Board of Supervisors are elected as representatives of specific districts within the city. Upon the death or resignation of mayor, the President of the Board of Supervisors becomes acting mayor until the full Board elects an interim replacement for the remainder of the term. In 1978, Dianne Feinstein assumed the office following the assassination of George Moscone and was later selected by the board to finish the term. In 2011, Edwin M. Lee was selected by the board to finish the term of Gavin Newsom, who resigned to take office as Lieutenant Governor of California. Lee (who won 2 elections to remain mayor) was temporarily replaced by San Francisco Board of Supervisors President London Breed after he died on December 12, 2017.  Supervisor Mark Farrell was appointed by the Board of Supervisors to finish Lee's term on January 23, 2018.  answer the following question:  What is the name of the county and that seceded from another county in 1856?
San Francisco