The Board of Supervisors on Tuesday approved a $537,000 grant to the Marin Agricultural Land Trust to help fund the purchase of a conservation easement near Tomales that will extend a continuous belt of 19 protected ranches. The development rights to the Thacher Ranch will total about $1.8 million dollars, with the difference funded by other grants and private donations. The 623-acre property has been in one family since 1954, but it is now undergoing…